babel fails to build with Python 3.9.0a6:

=================================== FAILURES ===================================
_ test_parse_future[\nfrom __future__ import print_function,\n    division, with_statement,\n    unicode_literals\n-237568] _
source = '\nfrom __future__ import print_function,\n    division, with_statement,\n    unicode_literals\n'
result = 237568
    @pytest.mark.parametrize('source, result', [
        ('''
    from __future__ import print_function,
        division, with_statement,
        unicode_literals
    ''', 0x10000 | 0x2000 | 0x8000 | 0x20000),
        ('''
    from __future__ import print_function, division
    print('hello')
    ''', 0x10000 | 0x2000),
        ('''
    from __future__ import print_function, division, unknown,,,,,
    print 'hello'
    ''', 0x10000 | 0x2000),
        ('''
    from __future__ import (
        print_function,
        division)
    ''', 0x10000 | 0x2000),
        ('''
    from __future__ import \\
        print_function, \\
        division
    ''', 0x10000 | 0x2000),
    ])
    def test_parse_future(source, result):
        fp = BytesIO(source.encode('latin-1'))
        flags = parse_future_flags(fp)
>       assert flags == result
E       assert 3801088 == 237568
tests/test_util.py:96: AssertionError
_ test_parse_future[\nfrom __future__ import print_function, division\nprint('hello')\n-73728] _
source = "\nfrom __future__ import print_function, division\nprint('hello')\n"
result = 73728
    @pytest.mark.parametrize('source, result', [
        ('''
    from __future__ import print_function,
        division, with_statement,
        unicode_literals
    ''', 0x10000 | 0x2000 | 0x8000 | 0x20000),
        ('''
    from __future__ import print_function, division
    print('hello')
    ''', 0x10000 | 0x2000),
        ('''
    from __future__ import print_function, division, unknown,,,,,
    print 'hello'
    ''', 0x10000 | 0x2000),
        ('''
    from __future__ import (
        print_function,
        division)
    ''', 0x10000 | 0x2000),
        ('''
    from __future__ import \\
        print_function, \\
        division
    ''', 0x10000 | 0x2000),
    ])
    def test_parse_future(source, result):
        fp = BytesIO(source.encode('latin-1'))
        flags = parse_future_flags(fp)
>       assert flags == result
E       assert 1179648 == 73728
tests/test_util.py:96: AssertionError
_ test_parse_future[\nfrom __future__ import print_function, division, unknown,,,,,\nprint 'hello'\n-73728] _
source = "\nfrom __future__ import print_function, division, unknown,,,,,\nprint 'hello'\n"
result = 73728
    @pytest.mark.parametrize('source, result', [
        ('''
    from __future__ import print_function,
        division, with_statement,
        unicode_literals
    ''', 0x10000 | 0x2000 | 0x8000 | 0x20000),
        ('''
    from __future__ import print_function, division
    print('hello')
    ''', 0x10000 | 0x2000),
        ('''
    from __future__ import print_function, division, unknown,,,,,
    print 'hello'
    ''', 0x10000 | 0x2000),
        ('''
    from __future__ import (
        print_function,
        division)
    ''', 0x10000 | 0x2000),
        ('''
    from __future__ import \\
        print_function, \\
        division
    ''', 0x10000 | 0x2000),
    ])
    def test_parse_future(source, result):
        fp = BytesIO(source.encode('latin-1'))
        flags = parse_future_flags(fp)
>       assert flags == result
E       assert 1179648 == 73728
tests/test_util.py:96: AssertionError
_ test_parse_future[\nfrom __future__ import (\n    print_function,\n    division)\n-73728] _
source = '\nfrom __future__ import (\n    print_function,\n    division)\n'
result = 73728
    @pytest.mark.parametrize('source, result', [
        ('''
    from __future__ import print_function,
        division, with_statement,
        unicode_literals
    ''', 0x10000 | 0x2000 | 0x8000 | 0x20000),
        ('''
    from __future__ import print_function, division
    print('hello')
    ''', 0x10000 | 0x2000),
        ('''
    from __future__ import print_function, division, unknown,,,,,
    print 'hello'
    ''', 0x10000 | 0x2000),
        ('''
    from __future__ import (
        print_function,
        division)
    ''', 0x10000 | 0x2000),
        ('''
    from __future__ import \\
        print_function, \\
        division
    ''', 0x10000 | 0x2000),
    ])
    def test_parse_future(source, result):
        fp = BytesIO(source.encode('latin-1'))
        flags = parse_future_flags(fp)
>       assert flags == result
E       assert 1179648 == 73728
tests/test_util.py:96: AssertionError
_ test_parse_future[\nfrom __future__ import \\\n    print_function, \\\n    division\n-73728] _
source = '\nfrom __future__ import \\\n    print_function, \\\n    division\n'
result = 73728
    @pytest.mark.parametrize('source, result', [
        ('''
    from __future__ import print_function,
        division, with_statement,
        unicode_literals
    ''', 0x10000 | 0x2000 | 0x8000 | 0x20000),
        ('''
    from __future__ import print_function, division
    print('hello')
    ''', 0x10000 | 0x2000),
        ('''
    from __future__ import print_function, division, unknown,,,,,
    print 'hello'
    ''', 0x10000 | 0x2000),
        ('''
    from __future__ import (
        print_function,
        division)
    ''', 0x10000 | 0x2000),
        ('''
    from __future__ import \\
        print_function, \\
        division
    ''', 0x10000 | 0x2000),
    ])
    def test_parse_future(source, result):
        fp = BytesIO(source.encode('latin-1'))
        flags = parse_future_flags(fp)
>       assert flags == result
E       assert 1179648 == 73728
tests/test_util.py:96: AssertionError


For the build logs, see:
https://copr-be.cloud.fedoraproject.org/results/@python/python3.9/fedora-rawhide-x86_64/01365332-babel/

For all our attempts to build babel with Python 3.9, see:
https://copr.fedorainfracloud.org/coprs/g/python/python3.9/package/babel/

Testing and mass rebuild of packages is happening in copr. You can follow these instructions to test locally in mock if your package builds with Python 3.9:
https://copr.fedorainfracloud.org/coprs/g/python/python3.9/

Let us know here if you have any questions.

Python 3.9 will be included in Fedora 33. To make that update smoother, we're building Fedora packages with early pre-releases of Python 3.9.
A build failure prevents us from testing all dependent packages (transitive [Build]Requires), so if this package is required a lot, it's important for us to get it fixed soon.
We'd appreciate help from the people who know this package best, but if you don't want to work on this now, let us know so we can try to work around it on our side.