Bump mypy to 1.8.0

[AlexWaygood]

The new mypy version flags a minor typing error in clinic.py:

Tools\clinic\clinic.py:5933: error: Subclass of "Sentinels" and "bool" cannot exist: "bool" is final  [unreachable]
                        if isinstance(value, (bool, NoneType)):
                                      ^~~~~
Tools\clinic\clinic.py:5933: error: Subclass of "Sentinels" and "NoneType" cannot exist: "NoneType" is final  [unreachable]
                        if isinstance(value, (bool, NoneType)):
                                      ^~~~~
Tools\clinic\clinic.py:5933: error: Subclass of "Null" and "bool" cannot exist: "bool" is final  [unreachable]
                        if isinstance(value, (bool, NoneType)):
                                      ^~~~~
Tools\clinic\clinic.py:5933: error: Subclass of "Null" and "NoneType" cannot exist: "NoneType" is final  [unreachable]
                        if isinstance(value, (bool, NoneType)):
                                      ^~~~~
Tools\clinic\clinic.py:5934: error: Statement is unreachable  [unreachable]
                            c_default = "Py_" + py_default
                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The issue is that our current annotation for value is incorrect; it should have a much broader type, since it could potentially be anything returned from a call to eval() on line 5927

[erlend-aasland]

Nice, thanks!