Improve PyPI packaging (#71)

* Move files for better setup.py packaging

* Update setup.py and configs for src layout

* Run tests on setup.py build

In effect, this tests that the setup.py configuration is correct.

coverage combine and coverage:paths are added to keep the displayed
coverage paths as src/ssh_audit/*.py instead of
.tox/$envname/**/site-packages/ssh_audit/*.py

* Remove unnecessary encoding declarations

Python 3 defaults to UTF-8 encoding.
https://docs.python.org/3/reference/lexical_analysis.html#encoding-declarations

* Remove shebang from colorama type stubs

Shouldn't need to be an executable.
Related: git has this file tracked as chmod -x.

test/stubs/colorama.pyi

@@ -1,5 +1,3 @@
-#!/usr/bin/env python
-# -*- coding: utf-8 -*-
 from typing import Optional
 
 def init(autoreset: bool = False, convert: Optional[bool] = None, strip: Optional[bool] = None, wrap: bool = True) -> None: ...