SCons: Format buildsystem files with psf/black

[akien-mga]

Configured for a max line length of 120 characters:

find \( -name "SConstruct" -o -name "SCsub" -o -name "*.py" \) -exec black -l 120 {} \;

psf/black is very opinionated and purposely doesn't leave much room for
configuration. The output is mostly OK so that should be fine for us,
but some things worth noting:

• Manually wrapped strings will be reflowed, so by using a line length
  of 120 for the sake of preserving readability for our long command
  calls, it also means that some manually wrapped strings are back on
  the same line and should be manually merged again.

• Code generators using string concatenation extensively look awful,
  since black puts each operand on a single line. We need to refactor
  these generators to use more pythonic string formatting, for which
  many options are available (%, format or f-strings).

• CI checks and a pre-commit hook will be added to ensure that future
  buildsystem changes are well-formatted.

[akien-mga]

Review comments welcome to identify locations where the formatted output doesn't look good and could be improved (which means changing the code, as we can't configure the formatter beyond line length).

As I mentioned in the OP, code generators look ugly and should be refactored, but it's a significant amount of busy work so I'm leaving it for a future PR (or a contributor? ;)).

[Xrayez]

As I mentioned in the OP, code generators look ugly and should be refactored, but it's a significant amount of busy work so I'm leaving it for a future PR (or a contributor? ;)).

Worth noting SCons provides Textfile() and Substfile() builders which could likely help this to some extent.

[akien-mga]

Cherry-picked for 3.2.2.