Support reading python version from mise config

[dbowring]

Description:
Adds support for reading .mise.toml (docs) if given as python-version-file.

Supports the short and longs forms, E.g.

[tools]
python = "3.12.2"

[tools]
python = { version="3.12.2", virtualenv=".venv" }

Note that this does not support the array form, E.g.,

[tools]
# not supported
python = ["3.12.2", "3.11.8", "3.10.14", "3.9.19", "3.8.19"]

Related issue:
N/A

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.