DOC: Set language in Sphinx config to en

[adswa]

Starting with Sphinx version 5.0, the configuration for the doc's
language should not be None anymore and causes docbuild failures
otherwise (see datalad/datalad#6715). This change sets the language
specification to English (en)

[codecov-commenter]

Codecov Report

Merging #147 (d853ddd) into master (7155e5f) will not change coverage.
The diff coverage is n/a.

❗ Current head d853ddd differs from pull request most recent head e9cbd74. Consider uploading reports for the commit e9cbd74 to get more accurate results

@@           Coverage Diff           @@
##           master     #147   +/-   ##
=======================================
  Coverage   89.65%   89.65%           
=======================================
  Files          12       12           
  Lines         406      406           
=======================================
  Hits          364      364           
  Misses         42       42           

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[adswa]

(cancelled appveyor)

[loj]

Thanks for catching this! Looks good to me.

[mih]

Thanks!