Use GHA-installed PyPy3

[hugovk]

Fixes #4382.

Changes proposed in this pull request:

• Instead of downloading and installing PyPy ourselves, use the one now available from GHA: https://github.com/marketplace/actions/setup-python-environment
• Saves about a minute of download/install time from the build

Last master pass

Python 3.6.9 (1608da62bfc7, Dec 23 2019, 12:38:24)
       [PyPy 7.3.0 with MSC v.1911 32 bit]
...
================ 1357 passed, 76 skipped in 161.27s (0:02:41) =================

https://github.com/python-pillow/Pillow/runs/407161674?check_suite_focus=true#step:24:16

This PR

Python 3.6.9 (1608da62bfc7, Dec 23 2019, 12:38:24)
       [PyPy 7.3.0 with MSC v.1911 32 bit]
...
================ 1357 passed, 76 skipped in 162.05s (0:02:42) =================

https://github.com/hugovk/Pillow/runs/408857182?check_suite_focus=true#step:23:16

[radarhere]

Thanks!