Use python3 -P while running test and tutorials on CI #17

python/cpython#31542 has introduced a -P flag that does not add the current directory to sys.path by default, but the new flag is only available on python >= 3.11.

As soon as dolfinx/dolfinx:nightly moves to python >= 3.11 the new flag could be used as in 91429d0
and the current workaround in b424642 will be removed.