https://github.com/mcmtroffaes/sphinxcontrib-bibtex/issues/163

This is not only reproducible with python 3.8 but also with older Pythons. However it blocks the bootstrap of Python 3.8.

Right, it appears to be an issue with Sphinx 2, not with python 3.8.  The advice to change the master_doc setting to 'contents' is wrong; there is no file named doc/contents.rst.  The warning comes from this code in sphinx's config.py, lines 510 to 522:

def check_master_doc(app, env, added, changed, removed):
    # type: (Sphinx, BuildEnvironment, Set[str], Set[str], Set[str]) -> Set[str]
    """Adjust master_doc to 'contents' to support an old project which does not have
    no master_doc setting.
    """
    if (app.config.master_doc == 'index' and
            'index' not in app.project.docnames and
            'contents' in app.project.docnames):
        logger.warning(__('Since v2.0, Sphinx uses "index" as master_doc by default. '
                          'Please add "master_doc = \'contents\'" to your conf.py.'))
        app.config.master_doc = "contents"  # type: ignore

    return changed

"Does not have no"?  Double negatives are so hard to understand.  Do they mean "does not have any"?  But that's wrong.  The project does have a master_doc setting.  Furthermore, there *is* a file named doc/index.rst, and there *is not* a file named doc/contents.rst, so how did we end up with 'index' not in app.project.docnames, and with 'contents' in app.project.docnames?  That suggests that app.project.docnames got set by traversing the test directory, which is where the files named contents.rst are, but we're working with doc/conf.py in the code where the warning is thrown.

Are we sure this isn't a sphinx 2 bug?  It sure looks like one.

> Are we sure this isn't a sphinx 2 bug?

We are totally not sure. Thanks for looking into it.

> The project does have a master_doc setting.

Note that those are test failures, not doc build failures. Does your investigation apply to the tests?

https://src.fedoraproject.org/rpms/python-sphinxcontrib-bibtex/pull-request/2

Note that that fixes the immediate problem, but there are indeed Python 3.8 incompatibilities:

ERROR: test_issue62.test_local_bibliographies
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/usr/lib/python3.8/site-packages/sphinx_testing/util.py", line 171, in decorator
    return func(*(args + (app, status, warning)), **kwargs)
  File "/builddir/build/BUILD/sphinxcontrib-bibtex-e8fb75a5491c1e560fea1f18d58041ea177103c0/test/test_issue62.py", line 75, in test_local_bibliographies
    app.builder.build_all()
  File "/usr/lib/python3.8/site-packages/sphinx/builders/__init__.py", line 284, in build_all
    self.build(None, summary=__('all source files'), method='all')
  File "/usr/lib/python3.8/site-packages/sphinx/builders/__init__.py", line 339, in build
    updated_docnames = set(self.read())
  File "/usr/lib64/python3.8/contextlib.py", line 120, in __exit__
    next(self.gen)
  File "/usr/lib/python3.8/site-packages/sphinx/util/logging.py", line 230, in pending_warnings
    memhandler.flushTo(logger)
  File "/usr/lib/python3.8/site-packages/sphinx/util/logging.py", line 193, in flushTo
    logger.handle(record)
  File "/usr/lib64/python3.8/logging/__init__.py", line 1598, in handle
    self.callHandlers(record)
  File "/usr/lib64/python3.8/logging/__init__.py", line 1660, in callHandlers
    hdlr.handle(record)
  File "/usr/lib64/python3.8/logging/__init__.py", line 959, in handle
    rv = self.filter(record)
  File "/usr/lib64/python3.8/logging/__init__.py", line 820, in filter
    result = f.filter(record)
  File "/usr/lib/python3.8/site-packages/sphinx/util/logging.py", line 428, in filter
    raise SphinxWarning(message)
sphinx.errors.SphinxWarning: syntax error in :filter: expression; invalid node <_ast.Constant object at 0x7f30d74c32e0> in filter expression

Fix at https://github.com/mcmtroffaes/sphinxcontrib-bibtex/pull/168