Fix serialization of composite flag members

[jakezwang]

Iterating over a flag enum omits zero-valued members, composite members, and aliases. Using that iteration to serialize the definition leaves members out of the constructor arguments and in the state restored afterward, which can raise AttributeError: cannot reassign member.

Use __members__ for both steps. Tests cover Flag and IntFlag with zero, composite and aliased values, including reconstruction in a separate process and both pickle protocols.

Fixes #558.

Validation: full test suites on Python 3.8, 3.11, 3.13 and 3.14; pre-commit checks for the changed files; wheel and sdist validation.