gh-104683: Make Argument Clinic parser template strings class level members

[erlend-aasland]

What do you think of this? IMO, all these static strings pollute output_templates. I'm trying to improve the readability of that method.

[erlend-aasland]

Makes sense! You could annotate them with Final[str] to indicate to mypy that they're meant to be class-level "constants" that are never changed when accessed on the class or the instance -- they are set once, and then should be treated by the type checker as read-only after that

Neat!

[erlend-aasland]

Opened python/cpython#107556