Use the PEP-702 `@deprecated` decorator to mark deprecated methods and
classes where possible. This makes deprecation information available to
static type checkers like mypy.

This commit also adds the deprecation plugin from `mypypp` in order to
add early support for the feature to mypy.

This solves all static deprecation warnings discovered by the previous
commit, and wmultiple runtime-only warnings that have been around for a
while now.

Keeping instances of GenericElement and its subclasses hashable has
caused problems in the past, especially in the case of custom equality
operators. Those problems include ambiguity about what gets hashed, as
well as hashing over mutable attributes, which both can cause the object
in question to end up in an unexpected hash bucket in one way or
another, which in turn leads to extremely difficult to diagnose issues
downstream. This is why hashing is now deprecated for all elements.
Previously it was only deprecated for a limited number of object types,
where it was definitely known to cause problems; this change improves
consistency across the API and prevents unexpected TypeErrors if
unhashable objects suddenly end up in hashing contexts.