Chore: Fix tests on Python 3.9 and SQLAlchemy 1.3, by skipping them

We don't know which circumstances cause this problem. SQLAlchemy 1.3 is
EOL anyway, so we don't care too much.

sqlalchemy.exc.InvalidRequestError: When initializing mapper mapped class RootStore->root, expression 'ItemStore' failed to locate a name ('ItemStore'). If this is a class name, consider adding this relationship() to the <class 'tests.compiler_test.SqlAlchemyDDLCompilerTest.test_ddl_with_foreign_keys.<locals>.RootStore'> class after both dependent classes have been defined.

tests/compiler_test.py

@@ -18,6 +18,7 @@
 # However, if you have executed another commercial license agreement
 # with Crate these terms will supersede the license and you may use the
 # software solely pursuant to the terms of the relevant commercial agreement.
+import sys
 import warnings
 from textwrap import dedent
 from unittest import mock, skipIf, TestCase
@@ -288,6 +289,8 @@ def test_for_update(self):
 FakeCursor = MagicMock(name='FakeCursor', spec=Cursor)
 
 
+@skipIf(SA_VERSION < SA_1_4 and (3, 9) <= sys.version_info < (3, 10),
+        "SQLAlchemy 1.3 has problems with these test cases on Python 3.9")
 class CompilerTestCase(TestCase):
     """
     A base class for providing mocking infrastructure to validate the DDL compiler.