Add an optional dependencies category for `psycopg2`

[namurphy]

Whilst going carefully through the getting started page, I noticed that it's currently necessary to install psycopg2 with a separate pip install command.

I suggest adding an optional dependencies category (like maybe all?) that would let dbprocessing be installed with:

pip install dbprocessing[all]

Doing this that this would let dbprocessing set bounds on the allowed versions of psycopg2, and to run tests using this version to make sure it's interoperable with it.

[jtniehof]

Yes; I suggest we support both [all] and [postgresql] so there's a little granularity here (e.g. we probably want [all] to include any hypothetical testing dependencies, and Sphinx for docs).

[namurphy]

Yes; I suggest we support both [all] and [postgresql] so there's a little granularity here

Agreed! I started thinking that way about two minutes after I posted this issue. 😅 Granularity is better. I like grains. 🌾

(e.g. we probably want [all] to include any hypothetical testing dependencies, and Sphinx for docs).

A related possibility would be to create a dev dependency group (which is not user facing) that could have docs and tests subgroups. 🤔