do_cmake.sh: use python-3.9 with fedora version 33

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)

Fixes: https://tracker.ceph.com/issues/47971
Signed-off-by: Sunny Kumar <sunkumar@redhat.com>
(cherry picked from commit 2cc747cf4a55fb38a524ec12f6b6e5888fadf07a)