Clarify + validate python_version

The Python docs for python_version say:

Specifies the Python version used to parse and check the target program. The string should be in the format MAJOR.MINOR – for example 3.9. The default is the version of the Python interpreter used to run mypy.

My understanding was that you could even target a newer version of Python than the one that installed mypy, and that mypy implemented its own Python parser or something. But it seems like python_version only supports specifying older Python versions; i.e. only enables restricting syntax, not enabling syntax.

But what was even more confusing was that specifying --python-version 3.100 didn't show any errors.

I think the following changes would be good:

1. Clarify the documentation to say that python_version is upper bounded to the version of Python running mypy
2. Error if python_version is greater than the version of Python running mypy

Start with the documentation for python_version and the command-line handling of --python-version, then reproduce the lack of an error with --python-version 3.100. Done means the documentation states the upper bound and an out-of-range version is rejected.