How to install psycopg2 under virtualenv

Psycopg is the most popular PostgreSQL adapter for Python. I prefer PostgreSQL to MySQL. Django projects usually integrate with PostgreSQL. Firstly we need to install 2 packages: libpq-dev and python-dev for Debian/Ubuntu.
Use

sudo apt install libpq-dev python3-dev

or

sudo apt install libpq-dev python-dev

depends on Python version you want.
Now install psycopg2 to integrate to our Django projects.

pip install psycopg2

Ensure to activate the virtual environment before installing psycopg2:

source {$your-env-dir}/bin/activate