this has been an alias to python for a very long time but it cannot be removed without a deprecation period

this is going to need a long deprecation period since it's sorta subtle and usually not the user's fault and will need hook authors to (potentially) make updates

the plan is to do the following:

1. introduce the following in a minor release
   • migrate-config will autofix .pre-commit-config.yaml usages of language: python_venv (there isn't an equivalent migrate-manifest -- though users outnumber hook authors by several orders of magnitude)
2. introduce the following in a minor release
   • a warning is shown for configuration using the language: python_venv
   • a warning is shown for repos using language: python_venv (do this at install time so it only shows once as to not be super annoying for users who have no control)
   • a recommendation for hook authors to also set minimum_pre_commit_version to this version
3. a long time passes (typically my deprecation period has been 12-18+ months)
4. introduce the following in a major release
   - removal of the python_venv alias