python pip silent install

If the answer is always y:

yes | pip install <package>

A silent install is possible by using the quiet flag:

pip install somepackage --quiet

This hides installation messages.
As per its documentation, note that this option is additive, and can be specified up to 3 times to remove messages of increasing levels of importance (warning, error, critical).

Additionally, you may want to force "always yes" as per this answer, and/or an exists-action option for a default behaviour when multiple choices exist:

yes | pip install somepackage --quiet --exists-action ignore

This truly has its mouth shut!