python-pyasn: use the real upstream

The https://code.google.com/p/pyasn/ project is not really the real
upstream for PyASN, and at least not the upstream for the PyASN
implementation recommended by the PySNMP developers.

Instead, the real upstream is
https://pypi.python.org/packages/source/p/pyasn1/, which has had much
more regular releases than the other PyASN implementation.

Therefore, we switch to using this implementation, as recommended by
the PySNMP developers on http://pysnmp.sourceforge.net/download.html

.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>