Use CMake FindPython3 module, raises minimum CMake to 3.12

Gives us out-of-the-box future proof python detection (python-3.9 right now).
1 job for master in 26 minutes and 53 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #73143
linux kf5-qt5 qt5.12

00:26:53

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