Specify language as 'en' instead of None

[jakirkham]

Needed for Sphinx 5 compatibility. Should fix the following warning occurring in doc builds.

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

xref: sphinx-doc/sphinx#10481

[bdice]

@jakirkham Do we want to remove the declaration of language entirely, rather than set it to "en"? This comment seems to suggest deleting the variable, and several linked PRs to other projects took that approach. sphinx-doc/sphinx#10474 (comment)

[jakirkham]

Interpret that comment as simply saying language = None shouldn't be there, but it doesn't rule out specifying language correctly

After all "explicit is better than implicit" (Zen of Python ;)

[bdice]

@jakirkham You’re right. I thought “en” and undefined might have different semantics (with undefined being closer to the previous value of None) but that’s not the case. See sphinx-doc/sphinx#10067. Yes, explicit is better here.

[jakirkham]

@gpucibot merge