Make the extension work for Python 3.8+.

See https://docs.python.org/3/c-api/stable.html for background.

Accordingly, the build matrix no longer needs the Python version.

Change-Id: I36c0f0c399c54e1ea06011bbe1d4aa8afef4cf05
Reviewed-on: https://code-review.googlesource.com/c/re2/+/61871
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>

python/setup.py

@@ -74,6 +74,9 @@ def options():
     bdist_wheel['plat_name'] = os.environ['PLAT_NAME']
   except KeyError:
     pass
+  # The extension works for Python 3.8+.
+  # See https://docs.python.org/3/c-api/stable.html for background.
+  bdist_wheel['py_limited_api'] = 'cp38'
   return {'bdist_wheel': bdist_wheel}