Improve typing

[kellerza]

Description

Improve typing, especially for EntityCollection, as a Generic[T]

This uses comments for types as per PEP484 and likely the best we can do while keeping support for Python 2.7, even if Python 2 support has been removed from mypy ref

Using the type parameters in the docstrings has a mixed effect:

1. when specifying a type the type is used
2. when specifying a param the type is never used

While 1 has the drawback of not adding any text description

Example

Even members are typed

[vgrem]

This is huge improvement in terms of typing and the the option to keep support with Python 2.7
Much appreciated!