Build PyPy3 wheels for manylinux

[hugovk]

Changes proposed in this pull request:

• We're already testing PyPy3
• manylinux2014 can build wheels for PyPy3.7 7.3
• So let's build and release them

[codecov-commenter]

Codecov Report

Merging #475 (3038bab) into main (a5d5f31) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #475   +/-   ##
=======================================
  Coverage   88.96%   88.96%           
=======================================
  Files           6        6           
  Lines        1685     1685           
=======================================
  Hits         1499     1499           
  Misses        186      186           

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