[th/python-black] add a script for running python black formatter and drop unit test
add contrib/scripts/nm-python-black-format.sh
script for black formatting.
This goes along ./contrib/scripts/nm-code-format.sh
(for clang-format).
Also, drop make check-python-black
unit test, that would test black formatting.
Also, use the script for the gitlab-ci check-tree
test.