Add winkerberos as an alternative dependency for kerberos

Based on another PR:
cloudera#504

README.md

@@ -38,6 +38,7 @@ Optional:
 
 * `kerberos>=1.3.0` for Kerberos over HTTP support. This also requires Kerberos libraries
    to be installed on your system - see [System Kerberos](#system-kerberos)
+  * On Windows operating systems an alternative is 'winkerberos' which can be installed with pip
 
 * `pandas` for conversion to `DataFrame` objects; but see the [Ibis project][ibis] instead
 

impala/_thrift_api.py

@@ -442,7 +442,14 @@ def get_custom_headers(cookie_header, has_auth_cookie):
     elif auth_mechanism == 'GSSAPI':
         # For GSSAPI over http we need to dynamically generate custom request headers.
         def get_custom_headers(cookie_header, has_auth_cookie):
-            import kerberos
+            # Try importing kerberos, then winkerberos as fallback. Report the original exception for kerberos.
+            try:
+                import kerberos
+            except ImportError as original_ex:
+                try:
+                    import winkerberos as kerberos
+                except ImportError:
+                    raise original_ex
             custom_headers = {}
             if cookie_header:
                 log.debug('add cookies to HTTP header')

setup.py

@@ -44,6 +44,7 @@ def readme():
     install_requires=['six', 'bitarray', 'thrift==0.16.0', 'thrift_sasl==0.4.3'],
     extras_require={
         "kerberos": ['kerberos>=1.3.0'],
+        "winkerberos": ['winkerberos'],
     },
     keywords=('cloudera impala python hadoop sql hdfs mpp spark pydata '
               'pandas distributed db api pep 249 hive hiveserver2 hs2'),