Use str.removeprefix and str.removesuffix where relevant

[jace]

A recurring pattern:

text: str
if text.startswith('prefix'):
    text = text[len('prefix'):]

Python 3.9 has two new methods on str, removeprefix and removesuffix, that collapse these into a single line. They are safer as we usually hardcode the value of len(prefix) and linters will not catch a discrepancy.