Use `str.removeprefix` and `str.removesuffix` where relevant

[jace]

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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.