fix(pendulum): Make PendulumDateTime work with postgres

[torotil]

fixes #754

[kurtmckee]

Thanks for this fix, and for including a test case to demonstrate it!

[torotil]

… and thanks to you for merging this so quickly!

[torotil]

@kurtmckee I have now taken a more in-depth look at the code in enriched_datetime. To me it seems a bit overly complicated. I think the indirection using the EnrichedDate / EnrichedDateTime objects is not actually worth it. There is almost no code shared between those classes. I have made a draft of removing that (and updating the the docs). Are you interested in getting a PR in that direction? (The draft can be found here: master...torotil:sqlalchemy-utils:refactor-enriched_datetime)

I also discovered that the arrow implementation already handles both timezone-aware and naive objects while the pendulum implementation seems to only handle naive ones (especially after the changes in this PR). I suppose this would need more test cases on the pendulum side.

[kurtmckee]

PRs are welcome, but I hope that there will be others with more knowledge of the codebase that @kvesteri is able to bring on board for maintenance.