Enhance reading from .python-version

[krystof-k]

Enhance reading from .python-version file, matching the Pyenv behavior:

• resolves multiple versions from multiple lines
• handles pyenv-virtualenv pointers (e.g. 3.10/envs/virtualenv) (see pyenv/pyenv-virtualenv#472)
• ignores empty lines and lines starting with #
• trims whitespace

Closes #734 and supersedes #647.

Check list:

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

[maruthiaws0808]

steps:

• uses: actions/checkout@v4
• uses: actions/setup-python@v4 with: python-version: 'graalpy-22.3'
• run: python my_script.py