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 #808 (a2db59f) into master (a766650) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #808   +/-   ##
=======================================
  Coverage   97.16%   97.16%           
=======================================
  Files          32       32           
  Lines         953      953           
  Branches      133      133           
=======================================
  Hits          926      926           
  Misses         21       21           
  Partials        6        6           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a766650...a2db59f. Read the comment docs.

[aleksihakli]

Thank you @hugovk :)