v0.20.1

fix: use safe_issubclass to prevent TypeError on Python 3.13

Python 3.13 raises TypeError when issubclass() receives a
types.GenericAlias (e.g. dict[X, set[Y]]) as first argument,
while Python 3.12 silently returns False. This caused a 500 error
on /voyager/dot when a route's response_model resolves to a
parameterized generic type.

Bump version to 0.20.1.

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com