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 Don't use .python-version for auto-activation pyenv/pyenv-virtualenv#472)
• ignores empty lines and lines starting with #
• trims whitespace

Closes #734 and supersedes #647.

Check list:

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