feat(tox): new release environment

Install and run `zest.releaser`'s `fullrelease` command to generate,
and upload a new release of the distribution.

Author: gforcada

config/default/tox.ini.j2

@@ -313,3 +313,12 @@ commands =
 #  _your own configuration lines_
 #  """
 ##
+
+[testenv:release]
+description = create a new release
+skip_install = true
+deps =
+    zest.release[recommended]
+    -c %(constraints_file)s
+commands =
+    fullrelease