Officially Support Python 3.10

[dmashine]

Hello!
If i run a test with python3 -We error level, i got this Warning:

  File "/usr/local/lib/python3.7/dist-packages/markdown/__init__.py", line 29, in <module>
    from .core import Markdown, markdown, markdownFromFile  # noqa: E402
  File "/usr/local/lib/python3.7/dist-packages/markdown/core.py", line 26, in <module>
    from . import util
  File "/usr/local/lib/python3.7/dist-packages/markdown/util.py", line 87, in <module>
    INSTALLED_EXTENSIONS = metadata.entry_points().get('markdown.extensions', ())
  File "/usr/local/lib/python3.7/dist-packages/importlib_metadata/__init__.py", line 312, in get
    flake8_bypass(self._warn)()
DeprecationWarning: SelectableGroups dict interface is deprecated. Use select.

python version is Python 3.7.3
see changes in metadata.py

[mitya57]

So instead of

metadata.entry_points().get('markdown.extensions', ())

We are supposed to use

metadata.entry_points(group='markdown.extensions')

The new API is already available in the importlib_metadata backport on PyPI, but it will be included in Python standard library only since version 3.10:
https://docs.python.org/3.10/library/importlib.metadata.html#entry-points

[waylan]

The new API is already available in the importlib_metadata backport on PyPI, but it will be included in Python standard library only since version 3.10

Note that we are using the backport for Python 3.6 and 3.7. So we only need to address this for 3.8 and 3.9. I suppose we could just require the backport for those as well. We would need to limit the backport to newer versions only which include support for the change.

Regardless, we do not yet officially support Python 3.10, and won't until close in time to its final release, so there is no rush. However, a PR is welcome if someone want's to.

[johnthagen]

This warning just started triggering when we added Python 3.10 to our CI matrix. A new release that fixes this would be appreciated.

A stacktrace:

  File "/.tox/docker/lib/python3.10/site-packages/rest_framework/compat.py", line 71, in <module>
    import markdown
  File "/.tox/docker/lib/python3.10/site-packages/markdown/__init__.py", line 29, in <module>
    from .core import Markdown, markdown, markdownFromFile  # noqa: E402
  File "/.tox/docker/lib/python3.10/site-packages/markdown/core.py", line 26, in <module>
    from . import util
  File "/.tox/docker/lib/python3.10/site-packages/markdown/util.py", line 87, in <module>
    INSTALLED_EXTENSIONS = metadata.entry_points().get('markdown.extensions', ())
  File "/usr/local/lib/python3.10/importlib/metadata/__init__.py", line 400, in get
    self._warn()
DeprecationWarning: SelectableGroups dict interface is deprecated. Use select.

[waylan]

As Python 3.10 was released last month, we should officially add support. I've adding this to the 3.3.5 Milestone so we don't forget about it. Of course, a PR is welcome.