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 #460 (d08ed86) into master (9554dba) will increase coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #460      +/-   ##
==========================================
+ Coverage   82.11%   82.29%   +0.18%     
==========================================
  Files          15       15              
  Lines         548      548              
  Branches       93       93              
==========================================
+ Hits          450      451       +1     
  Misses         65       65              
+ Partials       33       32       -1     

Impacted Files	Coverage Δ
constance/apps.py	100.00% <0.00%> (+5.00%)	⬆️

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