.gitlab-ci.yml: require Python 3

Python 2's include files still contain the `register` keyword which is no longer allowed in C++17; using it is an error for clang.

.gitlab-ci.yml

 ---
+variables:
+  DUNECI_CMAKE_FLAGS: "-DDUNE_PYTHON_FORCE_PYTHON3=TRUE"
+
 before_script:
   - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-matrix-vector.git
   - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-solvers.git