Support `.python-version` with multiple versions for `pyenv` manager

[rarkins]

Discussed in https://github.com/renovatebot/renovate/discussions/31837

^{Originally posted by mkniewallner October 7, 2024}

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Renovate implements pyenv manager to update Python version defined in a .python-version file. Per pyenv documentation, it is also possible to set multiple Python versions in a single .python-version file, separated by newlines:

You can also specify multiple versions in a .python-version file by hand, separated by newlines. Lines starting with a # are ignored.

Would it make sense for Renovate to support that, by extracting all possible Python versions defined in the file? Currently, the content of the file is trimmed, assuming that only one version can be specified.

If it makes sense, I'm willing to implement support for it.