gh-99360: Avoid `exc=None; del exc` in bytecode where unnecessary

[sweeneyde]

Functional changes:

• DELETE_FAST now assumes its argument is bound; it no longer raises UnboundLocalError by itself.
• When the state of a local is uncertain, del x compiles to LOAD_FAST_CHECK x; POP_TOP; DELETE_FAST x
• At the end of an exception handler except E as exc: ..., the exc=None before the del exc is omitted when not needed.

TODO: more tests, docs, NEWS

• Issue: gh-99360