bug: installation may fail on Windows for Impala backend due to aggressive kerberos support in setup.py

[ZeroCool2u]

This is a subtle bug that was introduced by a previous PR of mine here, which was meant to simplify kerberos dependencies.

However, some updates to the Impyla library have caused this bug to become a bigger problem for folks trying to install Ibis with the Impala backend on Windows.

The source of the issue is that when an Impyla install is requested with kerberos support, pip install impyla[kerberos], Impyla now attempts to install the kerberos library here.

On *nix based platforms this is fine, but on Windows, this is a problem. The kerberos library does not seem to support Windows and based on my research the typical pattern of library authors is using winkerberos or kerberos based on the platform detected by Python in the libraries setup.py.

I think this could easily be resolved by modifying our setup.py the same way I've suggested in this Impyla issue.

[cpcloud]

I think this needs to be solved in impyla and not in ibis. The workaround is a pretty big maintenance headache.