Bug 1466746 - dh-python backport for wheezy; r=glandium

dh-python isn't available in Wheezy. Let's backport it so we can
build Python packages that use it.

Fortunately for us, the package builds without any modifications.
The only customization we need is to ensure our custom Python
packages are present in order to satisfy Build-Depends.

MozReview-Commit-ID: CqZtwvosA6K