Import the ABCs from 'collections.abc' instead of 'collections'

[Colin-b]

Import the ABCs from 'collections.abc' instead of 'collections' by default as it is deprecated since Python3.7, and in 3.8 it will stop working.

I considered that Python3 is the de-facto standard as Python2 support is being dropped in every other Python project and support ends at end of year. This is why I removed the OrderedDict sample and use a builtin dict instead of they are ordered since 3.6

I however kept Python2 support in the unlikely event that some projects are still upgrading flask-restplus dependency while not upgrading to Python3.

I did not add any additional test case as I assume the test suite runs with Python2 as well as 3.X, if this is not the case I am more than happy to remove the extra TODO and try/except blocks in this PR.

closes #727

[coveralls]

Coverage increased (+0.08%) to 96.89% when pulling 136da9d on Colin-b:bugfix/python_38_support into 9b9ad42 on noirbizarre:master.

[andreykurilin]

looks good to me

[ziirish]

Thanks