Explicitly set a language

[mjpieters]

What do these changes do?

Change the Sphinx language parameter from None to en, explicitly setting
the language.

All documentation is in English, and Sphinx 5 complains loudly if this isn't
set.

Related issue number

#733, a dependabot PR that upgrades Sphinx to 5.0.1. The CI lint step fails due
to the warning.

Checklist

• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[codecov]

Codecov Report

Merging #739 (dece339) into master (80174ca) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #739   +/-   ##
=======================================
  Coverage   99.73%   99.73%           
=======================================
  Files           4        4           
  Lines         750      750           
  Branches      212      212           
=======================================
  Hits          748      748           
  Misses          2        2           

Flag	Coverage Δ
unit	99.60% <ø> (ø)

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