Use typing.Self where applicable

[InSyncWithFoo]

Self should be preferred over "ParserElement" since the former is dedicated for exactly this purpose and may, in some cases, provides better insight than a non-discriminating ParserElement.

This change requires typing_extensions as a type-checking time dependency; it should not alter runtime behaviour, unless someone inspects those type hints at runtime.

[ptmcg]

At the moment, pyparsing has no required external run-time dependencies, unless you want to create railroad diagrams, which are typically only a dev-time requirement.

Self will be available from typing in 3.11 without any added dependencies. External dependencies that are version-sensitive are a major chore. I'm open to revisiting this if we get some compelling examples where Self will be significantly better.