For some weird reasons, you have packages content installed but the
package manager didn't mark the package as installed. That's not normal
(perhaps because you tried with pip?).
Anyway, force the install by passing the option "--force" to yaourt.
As a side comment, I usually don't recommend to install libraries with
pip on the system as the distribution packages might conflict with them
and also for the ease of maintenance. Instead, I prefer to use
virtualenvs (for me, with pew).