Specify language as 'en' instead of None

[jakirkham]

Needed for Sphinx 5 compatibility. Should fix warning on CI.

Warning, treated as error:
[88](https://github.com/zarr-developers/numcodecs/runs/6993269158?check_suite_focus=true#step:8:89)
Invalid configuration value found: 'language = None'. Update your configuration to a valid langauge code. Falling back to 'en' (English).

xref: sphinx-doc/sphinx#10481

---

• Unit tests and/or doctests in docstrings
• tox -e py39 passes locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• tox -e docs passes locally
• GitHub Actions CI passes
• Test coverage to 100% (Coveralls passes)

[joshmoore]

Considering the current URLs already have en regardless (https://numcodecs.readthedocs.io/en/stable/ vs. https://numcodecs--329.org.readthedocs.build/en/329/) I'd say, 👍