Fixes # (n/a)

Summary/Motivation:

Make it easier to use black and harder to accidentally mess it up

Changes proposed in this PR:

Add [tool.black] section to pyproject.toml. This will cause a command-line invocation like black . to correctly use Pyomo's black options without needing to set them yourself.

Notes:

• Black walks up the filesystem looking for this file starting from the file being formatted, so it will still work if you run it from an unrelated working directory
• You can also set the intended python version, but it infers this anyways based on the requires-python field under [project] above.
• The difference between exclude and extend-exclude is that with extend-exclude it also excludes gitignored files (which are the default excluded files). This is meaningful because some of our tests generate python code and leave the gitignored, non-black-compliant files behind.
• It's still possible to mess this up if you set conflicting command-line flags, as they override this. This is most likely to happen if you are using an ide integration and you set a conflicting option there (as this is almost certainly implemented by changing the flags).

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