ORM executemany returning

Build on #5401 to allow the ORM to take advanage
of executemany INSERT + RETURNING.

Implemented the feature

updated tests

to support INSERT DEFAULT VALUES, needed to come up with
a new syntax for compiler INSERT INTO table (anycol) VALUES (DEFAULT)
which can then be iterated out for executemany.

Added graceful degrade to plain executemany for PostgreSQL <= 8.2

Renamed EXECUTEMANY_DEFAULT to EXECUTEMANY_PLAIN

Fix issue where unicode identifiers or parameter names wouldn't
work with execute_values() under Py2K, because we have to
encode the statement and therefore have to encode the
insert_single_values_expr too.

Correct issue from #5401 to support executemany + return_defaults
for a PK that is explicitly pre-generated, meaning we aren't actually
getting RETURNING but need to return it from compiled_parameters.

Fixes: #5263
Change-Id: Id68e5c158c4f9ebc33b61c06a448907921c2a657

test/sql/test_insert.py

@@ -936,11 +936,25 @@ class EmptyTest(_InsertTestBase, fixtures.TablesTest, AssertsCompiledSQL):
         dialect = default.DefaultDialect()
         dialect.supports_empty_insert = dialect.supports_default_values = True
 
-        stmt = table1.insert().values({})  # hide from 2to3
+        stmt = table1.insert().values({})
         self.assert_compile(
             stmt, "INSERT INTO mytable DEFAULT VALUES", dialect=dialect
         )
 
+    def test_supports_empty_insert_true_executemany_mode(self):
+        table1 = self.tables.mytable
+
+        dialect = default.DefaultDialect()
+        dialect.supports_empty_insert = dialect.supports_default_values = True
+
+        stmt = table1.insert().values({})
+        self.assert_compile(
+            stmt,
+            "INSERT INTO mytable (myid) VALUES (DEFAULT)",
+            dialect=dialect,
+            for_executemany=True,
+        )
+
     def test_supports_empty_insert_false(self):
         table1 = self.tables.mytable