Black settings ask for python 3.8 compatible syntax but enterprise supports python 3.7

[aarchiba]

pyproject.toml has a setting to determine which version of Python black should produce syntax for; this should be the oldest version on which enterprise is expected to run, but it isn't:

https://github.com/nanograv/enterprise/blob/master/pyproject.toml#L3

As result, black could reformat our code in a way that doesn't run on our test suite.

[AaronDJohnson]

Personal opinion: We should drop support for Python 3.7 and move to support 3.11 instead since 3.7 reaches EoL in 4 months (https://endoflife.date/python)

[vhaasteren]

We removed 3.7 with #362