Previously, autotools would detect whether we have "black"
in the path. And if so, it would check formatting during `make check`.

That's problematic. When I run `./contrib/fedora/rpm/build_clean.sh -w test`
in certain cases, it would pick up black, but then fail with

  Traceback (most recent call last):
    File "/usr/bin/black", line 5, in <module>
      from black import patched_main
    File "/usr/lib/python3.6/site-packages/black.py", line 42, in <module>
      from attr import dataclass, evolve, Factory
  ModuleNotFoundError: No module named 'attr'
  make[3]: *** [Makefile:21658: check-python-black] Error 1

That's an installation error of black, but still, during package build
there is no need to check the formatting. We could export
`NMTST_SKIP_PYTHON_BLACK=1` to prevent it, but it's still unnecessary.

We check proper formatting in gitlab-ci. That is enough, it doesn't
need to run during `make check`. In particular, because `black .`
takes 1.5 seconds on my machine.