googleapis/python-pubsub#500 added type annotation to the codebase, but we only checked that static type analysis passes under pytype. A lot of people, on the other hand, use mypy, but mypy currently reports 97 errors.

If we want users to leverage our type annotations, we should make sure that the checks pass with mypy as well, and then we can also add py.typed file to declare the library type-checked.

An additional requirement of this issue is that a new nox session named mypy needs to be added and enabled by default.

Acceptance criteria:

• A new nox session named mypy is added, and it runs on every PR.
  process: make sure type annoations pass with mypy python-pubsub#542
• The hand-written layer passes type checks.
  process: make sure type annoations pass with mypy python-pubsub#542
• The autogenerated part of the library is included in mypy checks and produces no errors.
  BLOCKED by Generated code for Pub/Sub does not pass type checks with mypy gapic-generator-python#1092
• Code samples are checked with mypy and the check runs on every PR (process: add mypy session for samples python-pubsub#551).
  • Refactor the manual client to inherit from the generated class instead of dynamically injecting the latter's methods. Mypy otherwise cannot know about the dynamically added methods.
• The library declares itself as type-checked by adding a marker py.typed file (requires all above items to be completed first)