add black github action

[JackTemaki]

I added a github action to check for the code formatting, I am not sure if we keep the default settings so I also added a pyproject.toml to configure black if needed.

So open discussion:

• should we keep the weird default of 88 characters? or do we adjust this to the more common 120?
• should run black with a specific python version targeted? (If not, it will determine the version based on each file individually)

[curufinwe]

Currently the code is formatted with the default 88 char limit and I dont see large problems.
I'd go for python 3.6 for now. Rational: rwth-i6/returnn#487

[JackTemaki]

Okay everything is ready now, I fixed the black version, applied it, and also added a requirements.txt so that the correct version can be easily installed.

[curufinwe]

Messed up original commit, force pushed indended (rebased) version