[3.8] bpo-37253: Document PyCompilerFlags.cf_feature_version (GH-14019) by miss-islington · Pull Request #14035 · python/cpython

@vstinner @miss-islington

* Update PyCompilerFlags structure documentation.
* Document the new cf_feature_version field in the Changes in the C
  API section of the What's New in Python 3.8 doc.
(cherry picked from commit 2c9b498)

Co-authored-by: Victor Stinner <vstinner@redhat.com>