Fix test_conflicting_post_params to work on pytest 5 (#5305)

The non-contextmanager form of pytest.raises was removed in pytest 5.
http://doc.pytest.org/en/latest/deprecations.html#raises-warns-with-a-string-as-the-second-argument

It was used here to support Python < 2.7, but that is no longer needed.
#1503 (comment)

Fixes #5304

Author: hroncok

tests/test_requests.py

@@ -774,8 +774,10 @@ def __len__(self):
     def test_conflicting_post_params(self, httpbin):
         url = httpbin('post')
         with open('Pipfile') as f:
-            pytest.raises(ValueError, "requests.post(url, data='[{\"some\": \"data\"}]', files={'some': f})")
-            pytest.raises(ValueError, "requests.post(url, data=u('[{\"some\": \"data\"}]'), files={'some': f})")
+            with pytest.raises(ValueError):
+                requests.post(url, data='[{"some": "data"}]', files={'some': f})
+            with pytest.raises(ValueError):
+                requests.post(url, data=u('[{"some": "data"}]'), files={'some': f})
 
     def test_request_ok_set(self, httpbin):
         r = requests.get(httpbin('status', '404'))