feat: create Python typing stub

Since python 3.6, we can add type information to python variables, functions, etc... Having such type information specified can avoid many mistakes early before they lead to runtime error. Since this library wants to maintain its support for python 2.7, adding type annotations to the code in the python modules is infeasible. As a result, stubs can be implemented to provide type information while maintaining backward-compatibility.

More information, https://www.python.org/dev/peps/pep-0484/