Add type hints and check them with mypy

[michael-k]

The file py.typed makes the type hints available for use in other projects.

There are still a lot of Anys. In some of those cases Any is fine (__eq__, __ne__, …), but in others I wasn't sure if I can come up with a complete list of allowed values. Eg. what does Principal accept as resources? Maybe ↓?

from typing import TYPE_CHECKING, Union, Sequence

if TYPE_CHECKING:
    from troposphere import Ref
    from .iam import ARN as IAM_ARN

Resources = Union[str, "IAM_ARN", "Ref", Sequence[Union[str, "IAM_ARN", "Ref"]]

class Principal(AWSHelperFn):
    def __init__(self, principal: str, resources: Resources = None) -> None:
        ...

troposphere.Ref makes it hard/impossible to properly restrict the type hints because awacs does not depend on troposphere and then troposphere.Ref is just Any for all projects that use awacs without troposphere.

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Maybe declare (note in changelog) type hints as experimental in the next release to (formally) allow changes without the need of a major version bump.

[markpeek]

Thanks!