math/flint2: Add explicit Python build dependency (D29353)

This unbreaks the build when using devel/samurai.

It previously got it through ninja which has a run dependency on
Python.

CMake Error at /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:218 (message):
  Could NOT find PythonInterp (missing: PYTHON_EXECUTABLE)
Call Stack (most recent call first):
  /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:582 (_FPHSA_FAILURE_MESSAGE)
  /usr/local/share/cmake/Modules/FindPythonInterp.cmake:169 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  CMakeLists.txt:37 (find_package)

http://package18.nyi.freebsd.org/data/122amd64-default-foo/2021-04-04_07h37m38s/logs/errors/flint2-2.7.1.log

PR:		254678