build: Revise MANIFEST.in strategy to properly use prune

[matthewfeickert]

c.f. scikit-hep/pyhf#1449 for reference and motivation

Needed for PR #94

* Use `prune **` to remove all files from the sdist
   - c.f. https://packaging.python.org/guides/using-manifest-in/#manifest-in-commands
   - "Setuptools also has undocumented support for ** matching zero or more characters including forward slash, backslash, and colon."
* Manually include all "default" files for a sdist in MANIFEST.in
   - c.f. https://packaging.python.org/guides/using-manifest-in/#how-files-are-included-in-an-sdist
* Add list of sdist contents to package publishing CI
   - Allow for easier checking of the sdist contents in CI logs

[codecov]

Codecov Report

Merging #95 (6f6fa97) into master (0f8af18) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #95   +/-   ##
=======================================
  Coverage   60.69%   60.69%           
=======================================
  Files           2        2           
  Lines         201      201           
  Branches       36       36           
=======================================
  Hits          122      122           
  Misses         72       72           
  Partials        7        7           

Flag	Coverage Δ
unittests	60.69% <ø> (ø)

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