add pre-commit hook definition

[ilCatania]

Closes #1300. Allows running pytype as a pre-commit hook. The caveat is, pytype still needs to be installed in the virtual environment where the type checks are being performed.

To test this from your python project:

pip install pre-commit pytype
pre-commit try-repo --verbose --all-files https://github.com/ilCatania/pytype.git

and you should get an output similar to this:

[INFO] Initializing environment for https://github.com/ilCatania/pytype.git.
===============================================================================
Using config:
===============================================================================
repos:
-   repo: https://github.com/ilCatania/pytype.git
    rev: 5a1228e8b277c3446c50fdc3767bd5057a886405
    hooks:
    -   id: pytype
===============================================================================
Pytype..................................................................Success
===============================================================================

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[rchen152]

Thanks! Looks like the CLA check is failing. Could you take a look at the instructions here: https://github.com/google/pytype/pull/1301/checks?check_run_id=8825270510?

[ilCatania]

I've done that already, but doesn't seem to be working because my email address is private... I'll try again

[ilCatania]

I don't know what magically happened but the CLA check went through today @rchen152

[rchen152]

Thanks!