Fix missing Property name in error messages

Content of the MR:(GaudiConfig2 only)

• drop some Python 2 compatibility hacks
• fix a minor inconsistency in the exception reported when setting string properties from wrong types
• migrated the tests of GaudiConfig2 from nosetests to pytest to use gaudi_add_pytest (!1440 (merged))
• add a test to expose issue #265 (closed)
• fix for #265 (closed)

Closes #265 (closed)