Remove requirement to singledispatch for python >= 3.4

[jbvsmo]

It is very weird that this library is being used. It is 6 months old and zero stars on github (https://github.com/jaraco/singledispatch).
Also this lib emits a warning regarding collections.abc

The functionality was added to functools on python 3.4 and so this should be a conditional requirement for people stuck in older unsupported python versions. Same thing was done to enum so please do the same here.

[zsiciarz]

This has been addressed in #317 .

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