add pre-commit configuration including black and flake8

[ottobackwards]

same as zkg
initial run

[ckreibich]

Okay so the cause of the test failures is that reformatting the code pulled pylint instructions on a separate line, so pylint now fails and errors out. That's the reason for this ...

  /project/zeekscript/formatter.py:196:38: E1131: unsupported operand type(s) for | (unsupported-binary-operation)

... which in turn is likely the cause of the test failure.

You also need to ensure we keep git blame working. Take another look at what we did in the individual commits in zkg, see here and in particular this:

https://black.readthedocs.io/en/stable/guides/introducing_black_to_your_project.html#avoiding-ruining-git-blame

Also please separate the pre-commit tool buildup so it's easier to see in the commit hierarchy what changes resulted.

Nusthell: I meant "what we did in zkg" a bit more literally. :-)

[bbannier]

I have opened #50 to add a pre-commit setup similar to the one added here, though I didn't add flake8 hooks since they seemed to produce nothing interesting (I checked with the config from this PR).