Fix arrow python type

[tim-haselhoff]

Fix sqlalchemy returning datetime in python instead of arrow when using arrow

[benedikt-bartscher]

@kvesteri foremost: thanks for maintaining sqlalchemy-utils. We really enjoyed using it so far.

Is there any way we can get this in? I would love to help if there is anything left to do

[torotil]

I think that this whole part of sqlalchemy-utils needs some work (see #761) … and it seems that there is currently noone around to maintain this code properly (ie. with an opinion on its architecture).

Having said that I had luck getting a bug fix in (#754) by also providing a test case that demonstrates the issue.

[benedikt-bartscher]

We could add a small test for it if required. @tim-haselhoff wdyt?

[kurtmckee]

There appear to be several unnecessary changes here; the only thing that appears necessary is to add:

        @property
        def python_type(self):
            return Arrow

Please add a unit test to demonstrate that this is resolving a bug.

[kurtmckee]

This results in an ImportError if arrow isn't installed; the current architecture is to allow for imports within the sqlalchemy_utils package but only raise an exception if a class is used but the underlying dependency isn't installed.