Update package metadata and tox4 setup

The current tox setup results in an 'editable-wheel' install which,
while it works, generates unhelpful warnings.  Switch to the 'editable'
type which is what you got under tox <= 4.

While we are reducing noise in the logs, correctly setup
openstack_elections.templates as a module.  We need it as a module as
we rely on Jinja2 to be able to use the module loader to locate the
templates.

Change-Id: Icbdca2f72c0777c587d6f95b3fab1911be179f97

openstack_election/templates/__init__.py

@@ -0,0 +1 @@
+__all__ = []

setup.cfg

@@ -17,7 +17,9 @@ classifier =
     Programming Language :: Python :: 3.6
 
 [files]
-packages = openstack_election
+packages =
+    openstack_election
+    openstack_election.templates
 
 [entry_points]
 console_scripts =

tox.ini

@@ -2,10 +2,11 @@
 envlist = docs,linters,py3
 minversion = 3.1.0
 ignore_basepython_conflict=true
+package = editable
+skip_install = true
 
 [testenv]
 basepython = python3
-usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt