move alabaster to docs extras

[artofhuman]

• Any changes relevant to users are recorded in the CHANGELOG.md.
• The documentation has been updated, if necessary.

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Closes #598

[codecov-io]

Codecov Report

Merging #599 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #599   +/-   ##
=======================================
  Coverage   60.58%   60.58%           
=======================================
  Files          62       62           
  Lines       11337    11337           
  Branches     2005     2005           
=======================================
  Hits         6869     6869           
  Misses       3911     3911           
  Partials      557      557

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[tpazderka]

Thanks, I will merge after the bugfix release is ready.

[tpazderka]

Merged manually.