* Use NamedTemporaryFile instead of TemporaryFile because we must
have a name which is a text on Python 3. Otherwise, the test fails.
On Python 3, TemporaryFile uses an integer for the file name, the
file descriptor.
* Replace filter() and map() with list comprehensions to get a list
on Python 3.
* On Python 3, temporary files are open in binary mode so write a
byte string instead of a text string.
* tox.ini: enable all image tests on Python 3.
Partial-Implements: blueprint porting-python3
Change-Id: Ice7f119c040bbddeda10ed45d137bb7851764b8d
80ae31f461