pip uses incorrect cached package version, instead of the user-specified version

I need to install psycopg2 v2.4.1 specifically. I accidentally did:

pip install psycopg2

Instead of:

pip install psycopg2==2.4.1

That installs 2.4.4 instead of the earlier version.

Now even after I pip uninstall psycopg2 and attempt to reinstall with the correct version, it appears that pip is re-using the cache it downloaded the first time.

How can I force pip to clear out its download cache and use the specific version I'm including in the command?