Make the dependencies more like a standard Python project and hook up the optional dependencies to setuptools (#4298)

tox.ini

@@ -9,9 +9,6 @@ deps =
     junitxml
     coverage
 
-    # needed by some of the tests
-    lxml
-
     # cyptography 2.2 requires setuptools >= 18.5
     #
     # older versions of virtualenv (?) give us a virtualenv with the same
@@ -33,6 +30,7 @@ setenv =
 [testenv]
 deps =
     {[base]deps}
+extras = all
 
 whitelist_externals =
     sh