Block release if git unclean

[pepoluan]

What do these changes do?

Makes release.py block release process if git is unclean, preventing snippets that one forgot to shelf from tainting the release sdist & wheel

Are there changes in behavior for the user?

None. This is purely a packaging issue.

Related issue number

Prevents #365 from happening again.

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [x] tox testenvs have been executed in the following environments:
  • [x] Windows (10): qa
• [x] Add a news fragment into the NEWS.rst file

[codecov[bot]]

Codecov Report

Merging #366 (45db1ee) into master (4bd08bb) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #366   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines         1706      1706           
  Branches       310       310           
=========================================
  Hits          1706      1706           

Impacted Files	Coverage Δ
aiosmtpd/__init__.py	100.00% <100.00%> (ø)

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