Does uninstalling a package with "pip" also remove the dependent packages?

You can install and use the pip3-autoremove utility to remove a package plus unused dependencies.

# install pip3-autoremove
pip install pip3-autoremove
# remove "somepackage" plus its dependencies:
pip-autoremove somepackage -y

No, it doesn't uninstall the dependencies packages. It only removes the specified package:

$ pip install specloud
$ pip freeze # all the packages here are dependencies of specloud package

figleaf==0.6.1
nose==1.1.2
pinocchio==0.3
specloud==0.4.5

$ pip uninstall specloud
$ pip freeze

figleaf==0.6.1
nose==1.1.2
pinocchio==0.3

As you can see those packages are dependencies from specloud and they're still there, but not the specloud package itself.

As mentioned below, you can install and use the pip-autoremove utility to remove a package plus unused dependencies.

I've successfully removed dependencies of a package using this bash line:

for dep in $(pip show somepackage | grep Requires | sed 's/Requires: //g; s/,//g') ; do pip uninstall -y $dep ; done

this worked on pip 1.5.4

You may have a try for https://github.com/cls1991/pef. It will remove package with its all dependencies.