Install package as editable

Replace '-e {toxinidir}' with 'package = editable', this allows
adding the source code early in the process and avoid packages listed
in the deps section to pull in charm-tools from pypi.

tox.ini

@@ -5,8 +5,7 @@
 
 [tox]
 envlist = py3
-skipsdist = true
-download = true
+package = editable
 
 [testenv]
 install_command =  pip install {opts} {packages}
@@ -19,7 +18,6 @@ deps =
     coverage
     mock
     responses
-    -e {toxinidir}
     https://github.com/openstack-charmers/charm-templates-openstack/archive/master.zip#egg=charm_templates_openstack
     ipdb