Avoid direct __annotations__ access but use get_type_hints instead.

[copybara-service]

Avoid direct __annotations__ access but use get_type_hints instead.

Also we're not using inspect.get_annotations, not only because it is 3.10+ only, but also we want the additional conversion behavior of get_type_hints including:

• Resolving string/ForwardRef types
• Annotation inheritance
• Unwrapping typing.Annotated

However this is also supported in inspect.get_annotations(eval_str=True), and the decision could be changed in the future.

References:

• Document difference between typing.get_type_hints and inspect.get_annotations(eval_str=True) python/cpython#102405
• https://bugs.python.org/issue43817