Building the documentation usually requires building the library too for introspection, so you will need the same prerequisites. The only extra prerequisite is virtualenv: the packages needed to build the docs will be installed when building the env.
Build the env once with:
make env
Then you can build the documentation with:
make
Or the single targets:
make html make text
You should find the rendered documentation in the html dir and the text
file psycopg2.txt.