- Add shebang and proper quoting for variable checks ($1 → "$1")
- Fix version detection to use exact match (grep ^VERSION$ not substring)
- Implement --local flag to create project-local .venv instead of pyenv-virtualenv
- Automatically upgrades pip/setuptools/wheel and installs requirements.txt
Even if .python-version is empty, virtualenv with desired name could be
created somewhere in the system. So we need to delete it in any case.
If there is no such virtualenv, we are ignoring errors.
