There is a consistent %check failure in python-markdown:

======================================================================
ERROR: Failure: YAMLLoadWarning (calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.)
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Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/nose/loader.py", line 251, in generate
    for test in g():
  File "/builddir/build/BUILD/Markdown-2.6.11/python2/tests/__init__.py", line 141, in TestSyntax
    config = get_config(dir_name)
  File "/builddir/build/BUILD/Markdown-2.6.11/python2/tests/__init__.py", line 75, in get_config
    config = YamlConfig(defaults, os.path.join(dir_name, 'test.cfg'))
  File "/builddir/build/BUILD/Markdown-2.6.11/python2/tests/__init__.py", line 39, in __init__
    self._config = yaml.load(f)
  File "/usr/lib64/python2.7/site-packages/yaml/__init__.py", line 109, in load
    load_warning('load')
  File "/usr/lib64/python2.7/site-packages/yaml/__init__.py", line 55, in load_warning
    warnings.warn(message, YAMLLoadWarning, stacklevel=3)
YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.

======================================================================
ERROR: testExtensionConfigOption (tests.test_apis.TestCliOptionParsing)
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Traceback (most recent call last):
  File "/builddir/build/BUILD/Markdown-2.6.11/python2/tests/test_apis.py", line 717, in testExtensionConfigOption
    options, logging_level = parse_options(['-c', self.tempfile])
  File "/builddir/build/BUILD/Markdown-2.6.11/python2/markdown/__main__.py", line 88, in parse_options
    extension_configs = yaml.load(fp)
  File "/usr/lib64/python2.7/site-packages/yaml/__init__.py", line 109, in load
    load_warning('load')
  File "/usr/lib64/python2.7/site-packages/yaml/__init__.py", line 55, in load_warning
    warnings.warn(message, YAMLLoadWarning, stacklevel=3)
YAMLLoadWarning: Failed parsing extension config file: /tmp/tmpZpOzSE.yml

======================================================================
ERROR: testExtensonConfigOptionAsJSON (tests.test_apis.TestCliOptionParsing)
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Traceback (most recent call last):
  File "/builddir/build/BUILD/Markdown-2.6.11/python2/tests/test_apis.py", line 747, in testExtensonConfigOptionAsJSON
    options, logging_level = parse_options(['-c', self.tempfile])
  File "/builddir/build/BUILD/Markdown-2.6.11/python2/markdown/__main__.py", line 88, in parse_options
    extension_configs = yaml.load(fp)
  File "/usr/lib64/python2.7/site-packages/yaml/__init__.py", line 109, in load
    load_warning('load')
  File "/usr/lib64/python2.7/site-packages/yaml/__init__.py", line 55, in load_warning
    warnings.warn(message, YAMLLoadWarning, stacklevel=3)
YAMLLoadWarning: Failed parsing extension config file: /tmp/tmpUaj1Qq.yml

======================================================================
ERROR: testExtensonConfigOptionBadFormat (tests.test_apis.TestCliOptionParsing)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/builddir/build/BUILD/Markdown-2.6.11/python2/tests/test_apis.py", line 760, in testExtensonConfigOptionBadFormat
    self.assertRaises(yaml.YAMLError, parse_options, ['-c', self.tempfile])
  File "/usr/lib64/python2.7/unittest/case.py", line 511, in assertRaises
    callableObj(*args, **kwargs)
  File "/builddir/build/BUILD/Markdown-2.6.11/python2/markdown/__main__.py", line 88, in parse_options
    extension_configs = yaml.load(fp)
  File "/usr/lib64/python2.7/site-packages/yaml/__init__.py", line 109, in load
    load_warning('load')
  File "/usr/lib64/python2.7/site-packages/yaml/__init__.py", line 55, in load_warning
    warnings.warn(message, YAMLLoadWarning, stacklevel=3)
YAMLLoadWarning: Failed parsing extension config file: /tmp/tmpoBfUjw.yml

----------------------------------------------------------------------
Ran 135 tests in 0.275s


This is because updated PyYAML now does those new warnings.
I think the test suite should be adapted to not treat them as errors (as a workaround before markdown adapts to the new API).

This FTBFS is blocking the bootstrap of Python 3.8.