Make python arcfour_crypt more portable

[Alexander Bokovoy]

On Tue, 02 Feb 2016, Stefan Metzmacher wrote:
> Hi Alexander,
> 
> >> RHEL7 has only m2crypto.
> >> And I see python-cryptography only on SLE12.
> > No, python-cryptography is in RHEL7, starting with 7.2, and we are
> > trying to avoid using m2crypto and standardize on python-cryptography.
> > python-cryptography is a way froward to support both Python2 and
> > Python3 upstream-wise.
> 
> Ok, but we also need to support RHEL 7.0 and 7.1...
So, for those m2crypto fallback would continue working just fine.
 
> >> As my code is better than the current state, I'd suppose that
> >> we take it as is. This fixes a bug and should be backported to 4.3
> >> and 4.4.
> >>
> >> We can do any further improvement later e.g. for python3 support later
> >> in master.
> > I agree with this, however, I'll come up with additional patches for
> > 4.3/4.4 for python-cryptography as well by the time Samba will be
> > rebased in RHEL7.
> 
> Sure, can I push with your Reviewed-by: for now?
Yes, please do!

Thank you for looking into the issue!
-- 
/ Alexander Bokovoy