bpo-35766: compile(): rename feature_version parameter

[vstinner]

Rename compile() feature_version parameter to _feature_version and
convert it to a keyword-only parameter.

Update also test_type_comments to pass feature_version as a tuple.

https://bugs.python.org/issue35766

[vstinner]

I proposed this change to make the further changes that I proposed in PR #13992. The PR #13992 introduced an inconsistency between ast.parse() and compile() API for feature_version. So I propose to make compile() feature_version private. Anyway, the parameter only makes sense for ast.parse() anyway, and it's undocumented on purpose.

I skipped the NEWS entry since feature_version is not documented on purpose:
https://docs.python.org/dev/library/functions.html#compile

[ilevkivskyi]

Again, as I said I am fine with this.

[miss-islington]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[bedevere-bot]

GH-13995 is a backport of this pull request to the 3.8 branch.

[vstinner]

Note: I saw that importlib has a source_to_code() which has a keyword-only "private" parameter: parameter called "_optimize", and the "_" prefix in Python stands for "private thing". I reused the idea here ;-)

We might make the parameter public later if needed, but I would prefer to accept the feature_version=(3, 6) format in that case. Right now, it's not needed, so I just made it private ;-)