-(option1) Incase you encounter an error installing psycopg2,
try installing the binaries instead:
pip install psycopg2-binary
-or (option2) downgrade the header file libpq5
sudo aptitude install libpq-dev # aptitude is recommended since it is better when it comes to resolving dependencies
accept the solution that recommends downgrading libpq5(usually the second recommended action)
pip install psycopg2
sudo apt-get install -y libpq5 # undo the downgrade