Remove python-argparse dependency in powerline

According to this message from portage:

!!! The following installed packages are masked:
- virtual/python-argparse-1::gentoo (masked by: package.mask)
/usr/portage/profiles/package.mask:
# Michał Górny <mgorny@gentoo.org> (6 Jul 2014)
# (on behalf of python@gentoo.org)
# The modules provided by those virtuals are built-in in all currently
# supported Python implementations. If your ebuild depends on either
# of them, please just remove the dependency. Removal in 30 days.