Refactor IndentStack?

[AlexWaygood]

We have a bunch of assertions around IndentStack to check that self.margin is not None:

cpython/Tools/clinic/clinic.py

Line 4376 in 49f238e

assert self.margin is not None, "Cannot call .indent() before calling .infer()"

cpython/Tools/clinic/clinic.py

Line 4384 in 49f238e

assert self.margin is not None, "Cannot call .indent() before calling .infer()"

cpython/Tools/clinic/clinic.py

Line 5267 in 49f238e

assert self.indent.margin is not None, "self.margin.infer() has not yet been called to set the margin"

This is a bit of a code smell, but it's hard to immediately see how to fix it.