Add PyPy3.7 wheels

[mattip]

Add pypy3.7 wheels. At some point perhaps pillow should consider dropping python3.6 across all implementations.

[mattip]

If it is not too much trouble it would be nice to upload the missing pypy3.7 wheels to save building from source.

[hugovk]

We'll at least drop CPython 3.6 when it goes EOL in December 2021.

The PyPy3.7 builds are failing to download:

https://downloads.python.org/pypy/pypy3.7-v7.3.1-linux64.tar.bz2:
  2020-12-20 09:36:12 ERROR 404: Not Found.
  Error: Process completed with exit code 8.

Should it be 7.3.3?

https://morepypy.blogspot.com/

[mattip]

xref matthew-brett/multibuild#374 to update multibuild to a latest version that includes pypy3.7

[mattip]

CI is passing.

[radarhere]

If it is not too much trouble it would be nice to upload the missing pypy3.7 wheels to save building from source.

You don't just want to wait until the next Pillow release on January 2nd?

[mattip]

Waiting is fine. Thanks!