CMake: Fix build with CMake < 3.12
CMake: Fix build with CMake < 3.12

FindPython3 is available only since CMake-3.12, but PythonInterp (that
available for ages) is deprecated since that 3.12 release.

TelepathyQt/telepathy-qt,TelepathyQt/telepathy-qt,TelepathyIM/telepathy-qt,TelepathyQt/telepathy-qt,TelepathyIM/telepathy-qt,TelepathyIM/telepathy-qt,TelepathyIM/telepathy-qt,TelepathyQt/telepathy-qt,TelepathyIM/telepathy-qt