black: configuration

This adds necessary configuration for flake8 and black to work together.

This also sets the line length to 99, per the data here:

* https://github.com/spack/spack/pull/24718#issuecomment-876933636

Given the data and the spirit of black's 88-character limit, we set the limit to 99
characters for all of Spack, because:

* 99 is one less than 100, a nice round number, and all lines will fit in a
  100-character wide terminal (even when the text editor puts a \ at EOL).
* 99 is just past the knee the file size curve for packages, and it means that packages
  remain readable and not significantly longer than they are now.
* It doesn't seem to hurt core -- files in core might change length by a few percent but
  seem like they'll be mostly the same as before -- just a bit more roomy.

- [x] set line length to 99
- [x] remove most exceptions from `.flake8` and add the ones black cares about
- [x] add `[tool.black]` to `pyproject.toml`
- [x] make `black` run if available in `spack style --fix`

Co-Authored-By: Tom Scogland <tscogland@llnl.gov>

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