hub image: add sqlalchemy-cocroachdb dependency

[weisdd]

Based on jupyterhub's docs, it should be generally compatible with any sqlalchemy provider.

In our environment, we're relying on cocroachdb due to easier clusterization and scalability.
Aside from a tiny defect in jupyterhub (jupyterhub/jupyterhub#3484, it was not part of 1.4.1), which might not even be specific to cocroachdb (rather to the dummy authenticator), everything seems to work well.

Currently, we have to dynamically install sqlalchemy-cockroachdb upon container start. It'd be great to have it included in the base image.

P.S. If I got it right, I had to run the dependencies freeze --upgrade script to make sure all dependencies are added to the requirements.txt. That's why you can see two files updated instead of just one (requirements.in).

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[consideRatio]

@minrk I think it is reasonable to add this python library to the hub image while still considering that we want to avoid increasing complexity and such, do you agree?

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[minrk]

I think this is fine to do for db backends that folks use, since there can't be many. Authenticators are more of a slippery slope, so I opened #2265 so we can have a better story for less-used dependencies than always submitting patches to the base image.