Sphinx issue when building docs

[coudot]

Running Sphinx v5.0.2
WARNING: Invalid configuration value found: 'language = None'. Update your configuration to a valid langauge code. Falling back to 'en' (English).

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/self-service-password/envs/latest/lib/python3.7/site-packages/sphinx/cmd/build.py", line 275, in build_main
    args.tags, args.verbosity, args.jobs, args.keep_going)
  File "/home/docs/checkouts/readthedocs.org/user_builds/self-service-password/envs/latest/lib/python3.7/site-packages/sphinx/application.py", line 223, in __init__
    self.setup_extension(extension)
  File "/home/docs/checkouts/readthedocs.org/user_builds/self-service-password/envs/latest/lib/python3.7/site-packages/sphinx/application.py", line 380, in setup_extension
    self.registry.load_extension(self, extname)
  File "/home/docs/checkouts/readthedocs.org/user_builds/self-service-password/envs/latest/lib/python3.7/site-packages/sphinx/registry.py", line 430, in load_extension
    mod = import_module(extname)
  File "/home/docs/checkouts/readthedocs.org/user_builds/self-service-password/envs/latest/lib/python3.7/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/home/docs/checkouts/readthedocs.org/user_builds/self-service-password/envs/latest/lib/python3.7/site-packages/sphinxcontrib/openapi/__init__.py", line 13, in <module>
    from sphinxcontrib.openapi import renderers, directive
  File "/home/docs/checkouts/readthedocs.org/user_builds/self-service-password/envs/latest/lib/python3.7/site-packages/sphinxcontrib/openapi/renderers/__init__.py", line 4, in <module>
    from ._httpdomain_old import HttpdomainOldRenderer
  File "/home/docs/checkouts/readthedocs.org/user_builds/self-service-password/envs/latest/lib/python3.7/site-packages/sphinxcontrib/openapi/renderers/_httpdomain_old.py", line 6, in <module>
    from .. import openapi20, openapi30, utils
  File "/home/docs/checkouts/readthedocs.org/user_builds/self-service-password/envs/latest/lib/python3.7/site-packages/sphinxcontrib/openapi/openapi20.py", line 16, in <module>
    from sphinxcontrib.openapi import utils
  File "/home/docs/checkouts/readthedocs.org/user_builds/self-service-password/envs/latest/lib/python3.7/site-packages/sphinxcontrib/openapi/utils.py", line 20, in <module>
    from m2r import convert as convert_markdown
  File "/home/docs/checkouts/readthedocs.org/user_builds/self-service-password/envs/latest/lib/python3.7/site-packages/m2r.py", line 59, in <module>
    class RestBlockGrammar(mistune.BlockGrammar):
AttributeError: module 'mistune' has no attribute 'BlockGrammar'

Exception occurred:
  File "/home/docs/checkouts/readthedocs.org/user_builds/self-service-password/envs/latest/lib/python3.7/site-packages/m2r.py", line 59, in <module>
    class RestBlockGrammar(mistune.BlockGrammar):
AttributeError: module 'mistune' has no attribute 'BlockGrammar'
The full traceback has been saved in /tmp/sphinx-err-agc99kfn.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!

Seems similar to Tribler/tribler#6624