TST: Refactor sql test inheritance

Pytest wouldn't discover the tests in the same way as nose when

1. The base class inherits from `TestCase`, and
2. The base class doesn't start with `Test` (e.g. PandasSQLTest)
3. The parent class with test methods doesn't start with `Test` (
  e.g. _TestSQLAPI)

We solved this by moving the class inheriting from `TestCase` down a
level from `PandasSQLTest` to the many `TestSQL*` methods.

pandas/io/tests/test_sql.py

@@ -236,7 +236,7 @@ def _close_conn(self):
         pass
 
 
-class PandasSQLTest(unittest.TestCase):
+class PandasSQLTest(object):
     """
     Base class with common private methods for SQLAlchemy and fallback cases.
 
@@ -839,7 +839,7 @@ def test_unicode_column_name(self):
         df.to_sql('test_unicode', self.conn, index=False)
 
 
-class TestSQLApi(SQLAlchemyMixIn, _TestSQLApi):
+class TestSQLApi(SQLAlchemyMixIn, _TestSQLApi, unittest.TestCase):
     """
     Test the public API as it would be used directly
 
@@ -1024,11 +1024,11 @@ def tearDown(self):
         super(_EngineToConnMixin, self).tearDown()
 
 
-class TestSQLApiConn(_EngineToConnMixin, TestSQLApi):
+class TestSQLApiConn(_EngineToConnMixin, TestSQLApi, unittest.TestCase):
     pass
 
 
-class TestSQLiteFallbackApi(SQLiteMixIn, _TestSQLApi):
+class TestSQLiteFallbackApi(SQLiteMixIn, _TestSQLApi, unittest.TestCase):
     """
     Test the public sqlite connection fallback API
 
@@ -1875,34 +1875,39 @@ def test_schema_support(self):
             tm.assert_frame_equal(res1, res2)
 
 
-class TestMySQLAlchemy(_TestMySQLAlchemy, _TestSQLAlchemy):
+class TestMySQLAlchemy(_TestMySQLAlchemy, _TestSQLAlchemy, unittest.TestCase):
     pass
 
 
-class TestMySQLAlchemyConn(_TestMySQLAlchemy, _TestSQLAlchemyConn):
+class TestMySQLAlchemyConn(_TestMySQLAlchemy, _TestSQLAlchemyConn,
+                           unittest.TestCase):
     pass
 
 
-class TestPostgreSQLAlchemy(_TestPostgreSQLAlchemy, _TestSQLAlchemy):
+class TestPostgreSQLAlchemy(_TestPostgreSQLAlchemy, _TestSQLAlchemy,
+                            unittest.TestCase):
     pass
 
 
-class TestPostgreSQLAlchemyConn(_TestPostgreSQLAlchemy, _TestSQLAlchemyConn):
+class TestPostgreSQLAlchemyConn(_TestPostgreSQLAlchemy, _TestSQLAlchemyConn,
+                                unittest.TestCase):
     pass
 
 
-class TestSQLiteAlchemy(_TestSQLiteAlchemy, _TestSQLAlchemy):
+class TestSQLiteAlchemy(_TestSQLiteAlchemy, _TestSQLAlchemy,
+                        unittest.TestCase):
     pass
 
 
-class TestSQLiteAlchemyConn(_TestSQLiteAlchemy, _TestSQLAlchemyConn):
+class TestSQLiteAlchemyConn(_TestSQLiteAlchemy, _TestSQLAlchemyConn,
+                            unittest.TestCase):
     pass
 
 
 # -----------------------------------------------------------------------------
 # -- Test Sqlite / MySQL fallback
 
-class TestSQLiteFallback(SQLiteMixIn, PandasSQLTest):
+class TestSQLiteFallback(SQLiteMixIn, PandasSQLTest, unittest.TestCase):
     """
     Test the fallback mode against an in-memory sqlite database.