Do not overwrite Python_ADDITIONAL_VERSIONS

[Flamefire]

Issue description

Currently the pybind11 CML sets Python_ADDITIONAL_VERSIONS no matter what. This leads to problems with the current (2.4.3) release when Python 3.8 is available next to some older Python. Normally a recentish CMake would find Python 3.8 just fine, but due to pybind11 setting Python_ADDITIONAL_VERSIONS to 3.7 ... it will search for 3.7 first (and finds the older version somewhere in the system)

A user could generically use cmake -DPython_ADDITIONAL_VERSIONS=3.8 ... to have any project search for this version first. But here pybind11 overwrites this so it has no effect.

Suggestion: Use list(APPEND Python_ADDITIONAL_VERSIONS 3.8 3.7 3.6 ...) This has been available since at least CMake 3.0 (can't find the docs for 2.8 easily but I'd recommend to switch to >=3.1 anyway. older ones have some weird behaviour corner cases)

[henryiii]

Fixed in #2338