Use winkerberos on windows

[jcrist]

This switches impyla to depend on winkerberos on windows rather than kerberos. The winkerberos package provides pre-built wheels for windows with the same python-facing API.

Fixes #466.

[csringhofer]

Thanks for working on this!
I agree that it would be much better to use a Kerberos module with prebuilt binaries.

I have concerns about testing though: I don't know whether anyone tests Impyla with Windows + Kerberos, so it seems possible to break some use cases without noticing it. All the test environments I know about are Linux based.
Are there relevant tests in the Ibis project?

[jcrist]

Are there relevant tests in the Ibis project?

There are not, and I also have no way to test this. I've had similar code in place for years in another kerberos-related project though with no complaints, but this may just mean that no one is using it on windows :).

This PR was quick to work up, no pressure at all to merge it. Apologies that I can't be more help here ensuring this is correct.

[csringhofer]

I started asking around about Impyla + Windows + Kerberos inside Cloudera.

I was thinking about an alternative in case switching to winkerberos from kerberos would be problematic for some people:

• the order of importing could reversed, so impyla would try winkerberos only if importing kerberos has failed
• a new extra name could be used, e.g. kerberos_with_winkerberos_on_windows or kerberos_prebuilt

This way an easy installation could be provided on Windows without the chance of breaking any existing use cases.

[csringhofer]

I started asking around about Impyla + Windows + Kerberos inside Cloudera.

I didn't get any feedback yet about this setup.
I am still concerned about possible breaking some existing workflows, so I would prefer to use the solution I wrote above, so first try importing kerberos and then winkerberos (if os.name == 'nt').

@jcrist Is it ok for you to proceed this way?