Operate on a copy of the read_data

This keeps us from mangling it if it contains lists, which we would be
popping items off of. We don't want to pop items from the list that was
passed in.

tests/support/mock.py

@@ -16,6 +16,7 @@
 
 from __future__ import absolute_import
 import collections
+import copy
 import errno
 import fnmatch
 import sys
@@ -331,6 +332,10 @@ class MockOpen(object):
       objects, representing the individual times that a given file was opened.
     '''
     def __init__(self, read_data=''):
+        # If the read_data contains lists, we will be popping it. So, don't
+        # modify the original value passed.
+        read_data = copy.copy(read_data)
+
         # Normalize read_data, Python 2 filehandles should never produce unicode
         # types on read.
         if not isinstance(read_data, dict):