tox: generic pluggymaster factor

tox.ini

@@ -42,11 +42,15 @@ setenv =
     pexpect: _PYTEST_TOX_PLATFORM=linux|darwin
     pexpect: _PYTEST_TOX_POSARGS_PEXPECT=testing/test_pdb.py testing/test_terminal.py testing/test_unittest.py
 
+    # TODO: remove?!
+    pluggymaster: _PYTEST_SETUP_SKIP_PLUGGY_DEP=1
+
     xdist: _PYTEST_TOX_POSARGS_XDIST=-n auto
 extras = testing
 deps =
     numpy: numpy
     pexpect: pexpect
+    pluggymaster: git+https://github.com/pytest-dev/pluggy.git@master
     xdist: pytest-xdist>=1.13
     {env:_PYTEST_TOX_EXTRA_DEP:}
 platform = {env:_PYTEST_TOX_PLATFORM:.*}
@@ -78,18 +82,6 @@ commands =
 deps = {[testenv:py27-trial]deps}
 commands = {[testenv:py27-trial]commands}
 
-[testenv:py27-pluggymaster]
-setenv=
-    {[testenv]setenv}
-    _PYTEST_SETUP_SKIP_PLUGGY_DEP=1
-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
 skipsdist = True