Python formatting

During today's telecon, it was (somewhat jokingly) suggested that we should run the Python formatting tool black on all Python scripts in the repository to guarantee a more consistent Python style. Since that is not very hard, I gave it a try.

Some surprises: some scripts apparently still used Python 2 syntax. So I ran 2to3 on those as well. Even then, there is one problematic script left:

error: cannot format ./examples/SubgridTests/CosmologicalStellarEvolution/plot_box_evolution.py: cannot use --safe with this file; failed to parse source file with Python 3.6's builtin AST. Re-run with --fast or stop using deprecated Python 2 syntax. AST error message: inconsistent use of tabs and spaces in indentation (<unknown>, line 111)

Is this something we want to do? And if so, do we want to include black formatting in format.sh?

If we want to proceed, the following issues need to be dealt with:

• fix examples/SubgridTests/CosmologicalStellarEvolution/plot_box_evolution.py formatting
• check that old Python 2 scripts were converted correctly and still do what they need to do