Add annotations and py.typed to conform with PEP561

[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

future changes to attrs_strict will need to ensure the stubs are updated appropriately as this will not come for free as long as we need to support python 2

[erikseulean]

This is great, thanks for the hard work. Could you please squash your commits ?

@gaborbernat since you have the most experience on this, I'd like to hear your comments on this.

[erikseulean]

Could you please capitalise the commit and add a description of what it does ? Helps with the docs...

[keiclone]

@erikseulean made a few changes. lemme know if they work?

[gaborbernat]

This should be not set here as we need to run against both python 2 and 3, given we support both.

[keiclone]

can remove this. we'd have to set two different runs in tox then, one for each version

[gaborbernat]

yeah, can do that in tox.ini as separate commands one after another

[gaborbernat]

we need some tool that validates that content from the python file matches with the stubs; for example https://pypi.org/project/retype/ is an example, please use it in our tox check... see https://github.com/ambv/retype/blob/master/tox.ini#L58

[keiclone]

wasn't aware of this. will add!

[erikseulean]

This will need rebase.