CI: Replace deprecated pypy3 with pypy-3.8

[hugovk]

pypy3 is deprecated and is not available in newer images:
actions/setup-python#244 (comment)

Instead explicitly specify the version:
https://github.com/actions/setup-python#specifying-a-pypy-version

Committed via https://github.com/asottile/all-repos

TODO

• Admin to replace this required branch check with build (pypy-3.8)

[codecov]

Codecov Report

Merging #157 (6ddc77e) into master (946541f) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #157   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          764       764           
  Branches       186       186           
=========================================
  Hits           764       764           

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[hugovk]

@beregond Please could you replace this required branch check with build (pypy-3.8)?

[beregond]

@hugovk yes, will do

[beregond]

Okay, can't help it myself - @jezdez maybe you can help with this?

[hugovk]

Updated to add a final, noop job to indicate success:

Its success depends on the build step fully passing. So instead of needing to adjust the checks each and every time a Python version is added or removed from the matrix, we can just add this new "test successful" as the required check.

@jezdez So please can you remove all the old "build" required checks and add "test successful"?

[jezdez]

@hugovk Done!