Development environment instructions should describe requirement for postgresql libraries

Without the postgresql libraries, `make devdata` fails when installing requirements. Specifically, installation of psycopg2 fails with errors that include:

```
Error: pg_config executable not found.

pg_config is required to build psycopg2 from source. Please add the directory
containing pg_config to the $PATH or specify the full executable path with the
option:

python setup.py build_ext --pg-config /path/to/pg_config build ...

or with the pg_config option in 'setup.cfg'.

If you prefer to avoid building psycopg2 from source, please install the PyPI
'psycopg2-binary' package instead.

For further information please check the 'doc/src/install.rst' file (also at
).
```

There are potentially a couple of solutions:
* Document that postgresql libraries are required for installation (postgresql-dev, libpq-dev, or whatever the distribution in question calls for.
* Depend on psycopg2-binary instead as the error message suggests. This might necessarily be as easy as it seems, since it's not just a direct specified dependency but I think may also be a transitive dependency.
* Upgrade to the psycopg package (the new name given to "psycopg3"), which does not appear to have this problem. That's probably the best idea in the long run since version 2 of psycopg is deprecated, but may involve other updates, too.