Python module install path wrong on MinGW

MinGW (at least the MSYS2 one) Python is a bit special and also uses the regular Unix location, so [the condition should probably be `if(WIN32 AND NOT MINGW)`](https://github.com/AcademySoftwareFoundation/OpenColorIO/blob/cac1ba5feffa8a59ddd072f4fd8db62c795acf64/src/bindings/python/CMakeLists.txt#L230).

Even better the block could be [replaced by something that figures out the location form `Python_SITEARCH`](https://github.com/AcademySoftwareFoundation/Imath/blob/0dc6d0a7cbae9eb8421e7d9944ce220ddab4d13d/src/python/CMakeLists.txt#L123) (which seems to be absolute, just need to extract the relative part).