Document how to install postgres support

[aholmes]

Currently, postgres support requires running pip install BL_Python.database[postgres] or pip install BL_Python.database[postgres-binary]. Document this. Or, is there a better way to go about it?

The reason it is an optional dependency is because the psycopg2 binary must be built, or the pre-built binary must be used. Some machines can build the binary without issue while others require some extra steps. The intent is to make this easier for users.

[aholmes]

The Makefile attempts to install the correct psycopg2 dependency. make $(SETUP_DEV_SENTINEL) and make $(SETUP_CICD_SENTINEL) will install the src/database[postgres-binary] dependency, which installs psycopg2-binary. Similarly, make $(PACKAGES) will install BL_Python.database[postgres-binary] if the target is BL_Python.database. There are no checks that the package works correctly; it is assumed that the correct binary package will be installed by pip.

However, in another project (internal name "CAP") we have gone further and verified the installation works and tried to resolve installation problems with it. This project installs BL_Python.database[postgres-binary] (per its pyproject.toml dependencies). This happens automatically when make is run (through the chain of prereqs).

 validate-psycopg2-install: $(VENV) $(SETUP_DEV_BACKEND_SENTINEL)
     @$(ACTIVATE_VENV) && \
     if ! python3 -c 'import psycopg2'; then \
         pip install psycopg2-binary --no-cache-dir --force-reinstall; \
     fi ; \
     if ! python3 -c 'import psycopg2'; then \
         echo "psycopg2 is not properly installed in the venv at $(VENV)"; \
     else \
         echo "psycopg2 is installed and working in the venv at $(VENV)."; \
     fi

Note that we have thus far intentionally avoided compiling psycopg2 on development machines because many of them lack the libxml2 dependencies.