Add types and type checking so mypy type checking is happy

[keithel-qt]

Mypy is presently not happy with sshtunnel, as it is not annotated with python types in it's variables and function signatures.

If you try importing sshtunnel in a project, and you run mypy against it, you will get messages similar to this:

myproject/ssh_tunnel.py:2: error: Skipping analyzing "sshtunnel": module is installed, but missing library stubs or py.typed marker
myproject/main.py:13: error: Skipping analyzing "sshtunnel": module is installed, but missing library stubs or py.typed marker
myproject/main.py:13: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 2 errors in 2 files (checked 32 source files)
[cargo-make] ERROR - Error while executing command, exit code: 1
[cargo-make] WARN - Build Failed.

I created this issue to track progress of sshtunnel working well with Mypy - being able to check sshtunnel's static typing.
This also would encompass adding static typing to sshtunnel, which will aid testing.