tox: Don't duplicate setup.py on which for-tests dependencies we need

-> Use .[test] to refer to them.
https://stackoverflow.com/a/41398850/9456786

/reviewed-by @jerome
/reviewed-on nexedi/zodbtools!14

tox.ini

@@ -4,11 +4,7 @@ envlist = py27-ZODB3, py{27,36,37}-ZODB{4,5,devPR183}
 
 [testenv]
 deps	=
-    # XXX tox does not consult extras_require['test'] -> install test dependencies explicitly
-    pytest
-    freezegun
-    pytz
-    mock;python_version<="2.7"
+    .[test]
 
     # latest ZODB from ZODB3 series
     ZODB3:  ZODB3 >=3.10, <3.11dev