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

tox.ini

@@ -3,8 +3,7 @@ envlist = {py27,py36,py37,pypy,pypy3}-{thread,gevent}
 
 [testenv]
 deps    =
-    # why tox does not get it from extras_require['test'] ?
-    pytest
+    .[test]
 
 # gpython pre-imports installed golang, will get into conflict with
 # golang/ if we run pytest from pygolang worktree. Avoid that.