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.

.gitignore

@@ -1,5 +1,5 @@
 *~
-*.pyc
+*.py[cod]
 *.exe
 *.asc
 venv*/
@@ -9,11 +9,16 @@ venv*/
 .coverage*
 reports/
 .scannerwork/
-packages/sshaudit/LICENSE
-packages/sshaudit/README.md
-packages/sshaudit/sshaudit.py
-packages/parts/
-packages/prime/
-packages/snap/
-packages/stage/
-packages/ssh-audit_*.snap
+
+# PyPI packaging
+/build/
+/dist/
+*.egg-info/
+*.egg
+
+# Snap packaging
+/parts/
+/prime/
+/snap/
+/stage/
+/ssh-audit_*.snap