CI: Replace deprecated pypy3 with pypy-3.8

[hugovk]

pypy3 is deprecated and is not available in newer images: https://github.com/actions/setup-python/issues/244#issuecomment-925966022

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

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

[codecov[bot]]

Codecov Report

Merging #171 (bd812c7) into master (836e279) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #171   +/-   ##
=======================================
  Coverage   94.85%   94.85%           
=======================================
  Files          10       10           
  Lines         350      350           
  Branches       71       71           
=======================================
  Hits          332      332           
  Misses         11       11           
  Partials        7        7           

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[hugovk]

No, this PR is telling GitHub Actions which Python to install via actions/setup-python.

And then https://github.com/jazzband/geojson/blob/master/tox.ini#L7 says to run on pypy3, which is still found as the Python interpreter:

Run actions/setup-python@v2
Successfully setup PyPy 7.3.7
 with Python (3.8.12)
...
Run coverage run --branch --source=geojson setup.py test
  coverage run --branch --source=geojson setup.py test
  coverage xml
  shell: /usr/bin/bash -e {0}
  env:
    pythonLocation: /opt/hostedtoolcache/PyPy/3.8.12/x64/bin

https://github.com/jazzband/geojson/runs/4170386960?check_suite_focus=true

[hugovk]

Rebased to include https://github.com/jazzband/geojson/pull/172.