do_cmake.sh: use python-3.9 with fedora version 33
Fedora-33 (rawhide) now has python-3.9.
Build breaks on Fedoa-33 with Traceback:
CMake Error at /usr/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:165 (message):
Could NOT find Python3: Found unsuitable version "3.9.0", but required is
exact version "3.8" (found /usr/bin/python3, found components: Interpreter
Development)