Issue #21750: mock_open.read_data can now be read from each instance, as it

could in Python 3.3.

diff --cc Lib/unittest/mock.py
Simple merge

diff --cc Lib/unittest/test/testmock/testmock.py
Simple merge

diff --cc Misc/NEWS
index 052e7ce48c177249f3b2295c5bb6d88c3f8026fe,2c669f3bbb07d0fa49bf706e428f52533db98d45..514c75904171db8e3ba224f40980b0a1be3e0c5e
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -73,6 -66,21 +73,9 @@@ Core and Builtin
  Library
  -------
  
 -- Issue #23247: Fix a crash in the StreamWriter.reset() of CJK codecs.
 -
 -- Issue #18622: unittest.mock.mock_open().reset_mock would recurse infinitely.
 -  Patch from Nicola Palumbo and Laurent De Buyst.
 -
 -- Issue #24608: chunk.Chunk.read() now always returns bytes, not str.
 -
 -- Issue #18684: Fixed reading out of the buffer in the re module.
 -
 -- Issue #24259: tarfile now raises a ReadError if an archive is truncated
 -  inside a data segment.
 -
+ - Issue #21750: mock_open.read_data can now be read from each instance, as it
+   could in Python 3.3.
+ 
  - Issue #24552: Fix use after free in an error case of the _pickle module.
  
  - Issue #24514: tarfile now tolerates number fields consisting of only