As a general solution to install Python packages with binary dependencies [1] on Debian/Ubuntu:
sudo apt-get build-dep python-ldap
# installs system dependencies (but not the package itself)
pew workon my_virtualenv # enter your virtualenv
pip install python-ldap
You'll have to check the name of your Python package on Ubuntu versus PyPI. In this case they're the same.
Obviously doesn't work if the Python package is not in the Ubuntu repos.
[1] I learnt this trick when trying to pip install matplotlib on Ubuntu.