Avoid builtins that were removed in Python 3

- `file` was removed in favor of `open`. Switch to the new spelling.
- `buffer` was removed in favor of a buffer protocol (and memoryview),
  and `reload` was moved to importlib.
  Both are used in py2-only blocks, so just placate PyLint.

https://fedorahosted.org/freeipa/ticket/5623

Reviewed-By: Jan Cholasta <jcholast@redhat.com>
Reviewed-By: Martin Basti <mbasti@redhat.com>

ipalib/cli.py

@@ -40,7 +40,7 @@ if six.PY3:
     unicode = str
 
 if six.PY2:
-    reload(sys)
+    reload(sys)                         # pylint: disable=reload-builtin
     sys.setdefaultencoding('utf-8')     # pylint: disable=no-member
 
 from ipalib import frontend