upip: MaxOSX mkdir("/") returns EISDIR, and not EEXIST.

Unlike Linux, which return EEXIST. This MacOSX behavior is not documented
in the official documentation:
https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man2/mkdir.2.html
(EISDIR isn't described as possible error for mkdir()!), but other projects
had to apply similar workarounds, e.g.:
https://github.com/janestreet/core/issues/7 . The issue appear to happen
only for root directory path, but workaround is applied conservatively
just in case.

upip/upip.py

@@ -56,7 +56,7 @@ def _makedirs(name, mode=0o777):
             os.mkdir(s)
             ret = True
         except OSError as e:
-            if e.args[0] != errno.EEXIST:
+            if e.args[0] != errno.EEXIST and e.args[0] != errno.EISDIR:
                 raise
             ret = False
     return ret