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.

PACKAGING

@@ -0,0 +1,47 @@
+= PyPI =
+
+To create package and upload to test server:
+
+# apt install virtualenv
+$ virtualenv -p /usr/bin/python3 /tmp/pypi_upload
+$ cd /tmp/pypi_upload; source bin/activate
+$ pip3 install twine
+$ cp -R path/to/ssh-audit .
+$ cd ssh-audit
+$ make -f Makefile.pypi
+$ make -f Makefile.pypi uploadtest
+
+
+To download from test server and verify:
+
+$ virtualenv -p /usr/bin/python3 /tmp/pypi_test
+$ cd /tmp/pypi_test; source bin/activate
+$ pip3 install --index-url https://test.pypi.org/simple ssh-audit
+
+
+To upload to production server:
+
+$ cd /tmp/pypi_upload; source bin/activate
+$ cd ssh-audit
+$ make -f Makefile.pypi uploadprod
+
+
+To download from production server and verify:
+
+$ virtualenv -p /usr/bin/python3 /tmp/pypi_prod
+$ cd /tmp/pypi_prod; source bin/activate
+$ pip3 install ssh-audit
+
+----
+
+= Snap =
+
+To create the snap package, run a fully-updated Ubuntu Server 20.04 VM.
+
+As root, run:
+
+# lxd init
+
+Leave all options default.  Bump the version number in snapcraft.yaml.  Then run:
+
+# make -f Makefile.snap