Add annotations and py.typed to conform with PEP561. Add type checking to tox

[keiclone]

*Issue number of the reported bug or feature request: #24

Describe your changes
made attrs-strict PEP561 compatible by adding annotations and py.typed. This removes the missing import error from mypy when attempting to run static type checking on packages using attrs-strict

Testing performed
Was able to successfully run static type checking in a test project after installing attrs-strict from my branch.

when testing on a separate library:

# test.py
@attr.s
class FooClass(object):
    list_of_numbers = attr.ib(validator=type_validator(), type=List[int])


fc = FooClass([1, 2, 3, "four"])

# before changes
$ pipenv install attrs-strict
Installing git+https://github.com/bloomberg/attrs-strict
...
$ mypy test.py
error: Cannot find implementation or library stub for module named 'attrs_strict'
note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
error: List item 3 has incompatible type "str"; expected "int"

# after changes
$ pipenv install git+https://github.com/keiclone/attrs-strict#egg=attrs-strict
Installing git+https://github.com/keiclone/attrs-strict#egg=attrs-strict
...
$ mypy test.py
error: List item 3 has incompatible type "str"; expected "int"

Additional context
could use some extra attention on how this was added to tox to make sure it was done correctly

[keiclone]

@gaborbernat sorry for opening a new PR, I borked my rebase process and figured it'd be easier/less risky to start over.

One thing to note is that it seems mypy seems to have some strange behavior when we do things like compare typing.Type[Any] to typing.Union, specifically a "Nonoverlapping comparison". Seems like an edge case that we can ignore for now.

I think I addressed all the comments in #31. Could you please take a look?

[erikseulean]

Why is the none check dropped here ?

[gaborbernat]

Not sure to be fair; I assume given the type checker does not complain it's not needed? @keiclone knows probably better

[erikseulean]

Looks good, just the comment about the None check.

[keiclone]

The None check was effectively moved up to line 69. _get_base_type only takes in and returns typing.Type types.

[erikseulean]

Thanks for the effort put into this @keiclone . Much appreciated.

[gaborbernat]

@erikseulean want to do a new release?

[erikseulean]

Yes, feel free to trigger a new one.