Add contrib/pyzfs/setup.py to .gitignore

As of 9ef798b77, setup.py is now generated from setup.py.in, but
this file was never moved to the .gitignore. This patch simply
corrects this issue.

Reviewed-by: Neal Gompa <ngompa@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8268