python_version should default to the interpreter for python_executable

[lordmauve]

Feature

If python_version is omitted and python_executable is set, the Python version is determined by running python_executable.

Currently python_version defaults to the version of Python that is running mypy.

Pitch

This reduces the amount of duplicate configuration needed in some cases.

For example, if mypy is installed with pipx then to run it against a venv you would need

python_executable = ".venv/bin/python3.10"
python_version = "3.10"

We particularly have a desire to avoid duplicating the Python version into lots of places. The target state would be

python_executable = ".venv/bin/python3"

This means that changing the version of Python in use in a project just involves smashing and recreating the venv, instead of configuration changes in mypy.