CLN: cleanup tox.ini, remove stale pandas/setup.py, fix tox warning
I always find it hard to get Packaging issues right, i'd appreciate if someone 
would back me up on pandas/setup.py being superfluous.

I made sure that setup.py install/develop still works, and that introducing
a failed test is  caught by tox as you would expect,

Also see #2029 for much faster tox runs.
