Move CWSchemaTC to RQLExpressionTC The test actually doesn't require a cubicweb schema, except for testing EmailAdress which is out of scope of the test (testing RQLExpression.transform_has_permission()). Just move the test in existing RQLExpressionTC inheriting from TestCase and rename the tests to be more explicit.

[skeleton] Drop py27 tox environment Since we dropped py2 support from cubicweb, it does not make much sense to keep it in skeleton from now on.