remove CI for 2.6 python version
Change is_method definition to use types module
add assertRaisesRegex without 'p' to python 2.x versions for version 3 compatibility
fix relative import to be compatible with python3
replace iteritems by items and use functions attributes compatibles with python3
Use tests.TestCase instead unittest one and replace assertRaisesRegexp by assertRaisesRegex
remove unused requirement and specify liskov version