[Developer Experience] Add typing support

[yuce]

Typing support improves editor intellisense and also the correctness of the code. We won't be able to support Python 3.5+'s typing syntax until we drop support for Python < 3.5, but we can still use mypy to add typing for older Python 3 versions and Python 2.7.

Here's an example Python 2.7/3 code which annotates the value parameter as a string. Both PyCharm and VS Code recognize the typing and recognize value is a string without any settings or third party extensions.

class C:
    def __init__(self, value):
        # type: (str) -> None
        self.value = value

See also: https://github.com/hazelcast/hazelcast-python-client/issues/356