Fix Big Sur detection
Fix Big Sur detection

XCode's python  returns 11.0
Homebrew's python  returns 10.16

Fixes #311

Part-of: <https://gitlab.freedesktop.org/gstreamer/cerbero/-/merge_requests/661>

GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero,GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,nirbheek/cerbero,GStreamer/cerbero,nirbheek/cerbero