Documentation should be updated to for new namespace pkg support

[chrisphilip322]

#5691 and #5591 added support for importing type hints from namespace packages according to pep 561 but the documentation hasn't been updated to reflect it https://mypy.readthedocs.io/en/latest/installed_packages.html#using-pep-561-compatible-packages-with-mypy

Specifically :

Note that mypy does not support some more advanced import features, such as zip imports, namespace packages, and custom import hooks.

I suggest simply updating the docs to:

Note that mypy does not support some more advanced import features, such as zip imports, namespace packages, and custom import hooks.

[gvanrossum]

See #5749.

[chrisphilip322]

I was specifically referring to this part of the documentation. https://github.com/python/mypy/blame/master/docs/source/installed_packages.rst#L28 which was not touched by #5749

[gvanrossum]

Ah, good call! Thanks for the reminder.

[gvanrossum]

Done, rev 9fcb9db (and rev 15d2668 in the release branch).