mypy issues

[daniel-sanche]

Currently, mypy is just testing the google.cloud.firestore module, which is mostly empty. If changed to test google.cloud.firestore_v1, errors come up. We should resolve all typing issues with the library

One option is to completely wipe the type annotations and start fresh, slowly adding them back over time

Or, we can fix it one file at a time, as we touch different areas of the code

[daniel-sanche]

update: we are working on a couple new features, but this is a high priority item once those are in