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.

pyproject.toml

@@ -0,0 +1,7 @@
+[build-system]
+# https://pip.pypa.io/en/stable/reference/pip/#pep-517-and-518-support
+requires = [
+    "setuptools>=40.8.0",
+    "wheel"
+]
+build-backend = "setuptools.build_meta"