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

[codecov]

Codecov Report

Merging #194 (9959d75) into master (70c549d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #194   +/-   ##
=======================================
  Coverage   92.06%   92.06%           
=======================================
  Files          12       12           
  Lines         378      378           
=======================================
  Hits          348      348           
  Misses         30       30           

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