[CMake] Look for Python3 again in `SearchInstalledSoftware.cmake`

[guitargeek]

This commit essentially reverts a288ff9 from four years ago, which was also part of the multi-python machinery that is not necessary anymore after Python 2 support was dropped.

Also, make the choice of looked-up components consistent with what is actually required.

[github-actions[bot]]

Test Results

     9 files       9 suites   1d 17h 57m 53s :stopwatch:  2 634 tests  2 634 :white_check_mark: 0 :zzz: 0 :x: 22 323 runs  22 323 :white_check_mark: 0 :zzz: 0 :x:

Results for commit 5ee40a9e.

:recycle: This comment has been updated with latest results.

[guitargeek]

Thanks a lot for the review! Can you maybe review the corresponding PR in roottest too? Thanks! https://github.com/root-project/roottest/pull/1120