Fix Meta.tz stub annotation to bool | None - #1179

import msgspec

m = msgspec.Meta(tz=True)

is_tz_aware: bool = m.tz
# Type "int | None" is not assignable to declared type "bool"
# "int" is not assignable to "bool" (basedpyright reportAssignmentType)

print(f"m.tz = {m.tz!r}, runtime = {type(m.tz).__name__}")
# m.tz = True, runtime = bool