* Add dev dependencies mypy and typing
* Add makefile target `types-test`, not yet included in `test` target.
* Generate stubs for imported modules to avoid mypy complaining about missing types.
* Remove generated stubs as there are way too many and they cause tons of mess in the repo. Switched to ignoring untyped modules for now, to concentrate on Quay-only type checking.
* mypy config changed to ignore missing imports
* ignore property decorator as it is not supported by mypy
* mypy annotations for many configuration variables
* re-generate mypy_stubs directory as its necessary in some classes for base classes to prevent mypy errors
* util/registry/queuefile referred to non existent definition of Empty class in multiprocessing.queues
* ignore type checking for things like monkey patching and exported/re-imported objects that
mypy does not allow.
* Adjust mypy config to warn us about unreachable return paths and useless expressions.
* Add the __annotations__ property to INTERNAL_ONLY_PROPERTIES so that it is not part of the config schema testing
* Remove redundant dependencies `typing` and `typing-extensions` which are NOOP after Python 3.5
* Remove mypy-extensions which only provides a TypedDict implementation but has not been updated since 2019.
* updated mypy to 0.910 which requires all types packages to be installed manually.
* exclude local-dev from type checking until core team can suggest an outcome for __init__.py duplicate packages
* re-add typing dependency which will be needed until Python 3.9
* ignore .mypy_cache
* add mypy stub for features module to replace inline definitions
* import annotations eager evaluation in billing.py as it was required to reference a class declared later in the module.
* remove the type definition of V1ProtocolSteps/V2ProtocolSteps to make tox happy
