Simplify release workflow

[jwodder]

This eliminates the use of auto version (a failure in which led to a failed & unsolicited attempt at uploading to PyPI) in favor of triggering the build & upload via auto's exec plugin and its afterRelease hook.

[codecov]

Codecov Report

Merging #444 (76f4ab4) into master (1affad9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #444   +/-   ##
=======================================
  Coverage   82.34%   82.34%           
=======================================
  Files          55       55           
  Lines        5664     5664           
=======================================
  Hits         4664     4664           
  Misses       1000     1000           

Flag	Coverage Δ
unittests	82.34% <ø> (ø)

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[yarikoptic]

not verse in auto enough to review. If works (or you think @jwodder it should work) -- good with me

[jwodder]

@yarikoptic This setup has already been used for releases of datalad-installer and fscacher, and I've tested it in a test repository. It works.

[yarikoptic]

Thank you @jwodder !