BLD: fixup setup.py, initialize pipe variable to None
recent changes (fe9c0ae0) broke setup.py for some users:

https://github.com/pydata/pandas/issues/5867#issuecomment-31759183

ironically their is not an easy way to do this if you are a user. iow, you use git to clone the dir, then uninstall git????
or copy a cloned dir to another machine. weird.
 Actually, I completely botched this... there used to be a defensive try/except which swallowed this.
users that get a straight tarball are affected by this, and I'm afraid that means 0.13.0 is borked
as well for users who don't have git installed.

Windows users usually install binaries and others that use tarball will almost always have git
installed so not a complete clusterf**k.

never ever change working code. In fact, never write code. it always ends badly.

and naturally, the windows CI box has git installed... so this slipped through. _sigh_
 didnt' you change this after 0.13? 7c6e5f662684dbfefc5f2d4900df74c955ae95c7
 oh yeah, fooled by commit log ordered by author date rather then commit date.
I remember now, I waited till 0.13.0 to merge. Release paranoia wins out again.
 thanks for the report @ruidc
