Update setup.py syntax so RuntimeError is properly raised

[wbarnha]

Something I noticed is that if you tried running setup.py with older versions of Python, you'd see:

  File "setup.py", line 22
    f"{type(self).__name__} is forbidden, "
                                          ^
SyntaxError: invalid syntax

I made a minor adjustment so that if someone was running Python 2.7 for instance, the proper exception gets raised:

RuntimeError: support for python < 3.6 has been removed in setuptools_scm>=6.0.0

I know support for older Python versions is deprecated, but I think it'd be a good fix so other developers are more aware of their mistake.

[RonnyPfannschmidt]

the errors are from a new setuptools release, will investigate later

[RonnyPfannschmidt]

this requires a pre-commit exclude for the pyupgrade of setup.py