Add type hints to the library API

[oleg-nenashev]

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Proposal

The Python runtime does not enforce function and variable type annotations, but they can be used by third party tools such as type checkers, IDEs, linters, etc. PEP-484 and 483 introduced a standard for typing annotations that are supported in modern Python IDEs, and mentioned as a part of the minimum quality bar for adoption

Scope

• [ ] Annotate the existing API with Types - likely a breaking change for 3.0
• [ ] Document the expectations in the contributing guide
• [ ] Enable the necessary linters in the project itself

References

• https://docs.python.org/3/library/typing.html