[Developer Experience] Add typing support

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](https://mypy.readthedocs.io/en/stable/) 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.

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

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