Support pytest 4

Fixes https://github.com/kennethreitz/requests/issues/5048

See https://docs.pytest.org/en/latest/deprecations.html#marks-in-pytest-mark-parametrize

setup.py

@@ -54,7 +54,7 @@ test_requirements = [
     'pytest-mock',
     'pytest-xdist',
     'PySocks>=1.5.6, !=1.5.7',
-    'pytest>=2.8.0'
+    'pytest>=3'
 ]
 
 about = {}

tests/test_utils.py

@@ -33,7 +33,8 @@ class TestSuperLen:
         'stream, value', (
             (StringIO.StringIO, 'Test'),
             (BytesIO, b'Test'),
-            pytest.mark.skipif('cStringIO is None')((cStringIO, 'Test')),
+            pytest.param(cStringIO, 'Test',
+                         marks=pytest.mark.skipif('cStringIO is None')),
         ))
     def test_io_streams(self, stream, value):
         """Ensures that we properly deal with different kinds of IO streams."""