posix: remove is-comparison between integers

Comparing integers by identity relies on a CPython implementation
detail of caching integers between -5 and 256.[1]

[1] <http://docs.python.org/c-api/int.html#PyInt_FromLong&gt;

mercurial/posix.py

@@ -144,7 +144,7 b" if sys.platform == 'darwin':"
         try:
             fd = os.open(path, O_SYMLINK)
         except OSError, err:
-            if err.errno is errno.ENOENT:
+            if err.errno == errno.ENOENT:
                 return path
             raise