Add compatibility with newer versions of pytest.

Taken from upstream:

  https://github.com/dateutil/dateutil/commit/2bdd63158b7f981fc6d70a869680451bdfd8d848

diff --git a/dateutil/test/test_internals.py b/dateutil/test/test_internals.py
index 53081314..b32e6723 100644
--- a/dateutil/test/test_internals.py
+++ b/dateutil/test/test_internals.py
@@ -9,6 +9,7 @@
 
 import sys
 import pytest
+import warnings
 
 from dateutil.parser._parser import _ymd
 from dateutil import tz
@@ -65,18 +66,17 @@ def test_parser_parser_private_not_warns():
     from dateutil.parser._parser import _timelex, _tzparser
     from dateutil.parser._parser import _parsetz
 
-    with pytest.warns(None) as recorder:
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
         _tzparser()
-        assert len(recorder) == 0
 
-    with pytest.warns(None) as recorder:
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
         _timelex('2014-03-03')
 
-        assert len(recorder) == 0
-
-    with pytest.warns(None) as recorder:
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
         _parsetz('+05:00')
-        assert len(recorder) == 0
 
 
 @pytest.mark.tzstr
f0299e61'>tree</a><a href='/guix/commit/COPYING?id=c16add7fd9783db46bb5b308a885af62f0299e61'>commit</a><a href='/guix/diff/COPYING?id=c16add7fd9783db46bb5b308a885af62f0299e61'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/COPYING'>
<input type='hidden' name='id' value='c16add7fd9783db46bb5b308a885af62f0299e61'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=c16add7fd9783db46bb5b308a885af62f0299e61'>root</a>/<a href='/guix/log/COPYING?id=c16add7fd9783db46bb5b308a885af62f0299e61'>COPYING</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/COPYING?id=c16add7fd9783db46bb5b308a885af62f0299e61&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>