New option --python-version. - #883

I suggest you use mypy.syntax.dialect.Dialect from #880 instead. Of course, @JukkaL is inactive so making PRs is kind of pointless.

Instead of only a linear version, it computes the set of actual features, and can create variations. So, you would create one base version (e.g. Dialect(sys.version[:3])), and then from that create a version to allow stub-specific syntax in .pyi files but not .py files (currently just stub_dialect = dialect.add_future('mypy-codec') to allow annotations). Finally, as each file is parsed, it grows its own sub-dialect depending on the __future__ statements in effect.

Yes, your work in #880 looks highly relevant.
I primarily use pyversion to figure out which file to pull in, though. See the work in progress at matthiaskramm@bdaeea0 .
But of course applying the version to parsing is a secondary concern.

Note, #887 is cherry-picked from syntax, and also updated version to keep the patchlevel, and also support getting sys.path etc from an inferior.

This looks reasonable for now, and unblocks work on typeshed integration. #880 is probably some way off from being merged, it doesn't make sense to wait for that.

Note that I really am serious about #887 being ready even though #880 is not. It will almost certainly be needed for #732.