python-pyftpdlib: bump version to 1.5.1, fixing Python 3 issue

python-pyftpdlib: bump version to 1.5.1, fixing Python 3 issue

This commit bumpds the version of python-pyftpdlib to fix a
byte-compilation problem occuring with Python 3. The current version of
pyftpdlib contained a file that had mistakenly been added to the
release, and which wasn't Python 3 compliant. The new 1.5.1 version has
been released specifically to address this problem, see:

  https://github.com/giampaolo/pyftpdlib/issues/381

Until now, this wasn't causing any problem as a byte-compilation problem
was not a fatal failure. But upcoming commits will make it a fatal
failure, so it needs to be fixed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>