Convert a few more small test classes to be pytest-style (#585)

* Rewrite test_tsafe to be pytest-style

* Rewrite TestRevoked to be pytest-style

* Convert TestConnection to be pytest-style
diff --git a/tests/test_tsafe.py b/tests/test_tsafe.py
index 97045ce..8ffe35a 100644
--- a/tests/test_tsafe.py
+++ b/tests/test_tsafe.py
@@ -2,22 +2,20 @@
 # See LICENSE for details.
 
 """
-Unit tests for :py:obj:`OpenSSL.tsafe`.
+Unit tests for `OpenSSL.tsafe`.
 """
 
 from OpenSSL.SSL import TLSv1_METHOD, Context
 from OpenSSL.tsafe import Connection
 
-from .util import TestCase
 
-
-class ConnectionTest(TestCase):
+class TestConnection(object):
     """
-    Tests for :py:obj:`OpenSSL.tsafe.Connection`.
+    Tests for `OpenSSL.tsafe.Connection`.
     """
     def test_instantiation(self):
         """
-        :py:obj:`OpenSSL.tsafe.Connection` can be instantiated.
+        `OpenSSL.tsafe.Connection` can be instantiated.
         """
         # The following line should not throw an error.  This isn't an ideal
         # test.  It would be great to refactor the other Connection tests so