Fix lint in unit tests re: py3-first and py2 compat

Use local heler for now due to the transitional state of this charm
from py2 to py3, where cmp does not exist.

Change-Id: I86161b8886da69833c4cc074c0f7ede15f456c9d

hooks/percona_utils.py

@@ -405,10 +405,15 @@ def assert_charm_supports_ipv6():
                         "versions less than Trusty 14.04")
 
 
+def _cmp(x, y):
+    """Shim for py2 py3 compat."""
+    return (x > y) - (x < y)
+
+
 def unit_sorted(units):
     """Return a sorted list of unit names."""
     return sorted(
-        units, lambda a, b: cmp(int(a.split('/')[-1]), int(b.split('/')[-1])))
+        units, lambda a, b: _cmp(int(a.split('/')[-1]), int(b.split('/')[-1])))
 
 
 def install_mysql_ocf():

unit_tests/test_utils.py

@@ -130,7 +130,7 @@ def patch_open():
 
     Yields the mock for "open" and "file", respectively.'''
     mock_open = MagicMock(spec=open)
-    mock_file = MagicMock(spec=file)
+    mock_file = MagicMock(spec=__file__)
 
     @contextmanager
     def stub_open(*args, **kwargs):