tox: use deps for pluggymaster testenvs

tox-dev/tox#706 has been fixed.

Author: blueyed

tox.ini

@@ -116,11 +116,13 @@ commands = {[testenv:py27-numpy]commands}
 setenv=
     {[testenv]setenv}
     _PYTEST_SETUP_SKIP_PLUGGY_DEP=1
-    # NOTE: using env instead of "{[testenv]deps}", because of https://github.com/tox-dev/tox/issues/706.
-    _PYTEST_TOX_EXTRA_DEP=git+https://github.com/pytest-dev/pluggy.git@master
+deps =
+    {[testenv]deps}
+    git+https://github.com/pytest-dev/pluggy.git@master
 
 [testenv:py37-pluggymaster]
 setenv = {[testenv:py27-pluggymaster]setenv}
+deps = {[testenv:py27-pluggymaster]deps}
 
 [testenv:docs]
 basepython = python3