gh-104683: Argument clinic: Make the filename parameter to Clinic required

[AlexWaygood]

Clinic objects are only ever created in one place in clinic.py, and the filename parameter is passed an object that we know to be a string:

cpython/Tools/clinic/clinic.py

Line 2355 in 89fd4f4

clinic = Clinic(language, verify=verify, filename=filename)

There's no need for this parameter to be optional. By making it required, we can remove an ugly assertion elsewhere that we added to keep mypy happy.

• Issue: Modernise code in Tools/clinic/ #104683