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)

[codeclimate]

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[codecov]

Codecov Report

Merging #38 (2dab8fc) into master (0d19eed) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #38   +/-   ##
=======================================
  Coverage   93.02%   93.02%           
=======================================
  Files           4        4           
  Lines          43       43           
=======================================
  Hits           40       40           
  Misses          3        3           

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