Tests: Python 3 compatibility

The tests use '#!/usr/bin/env python' which may be Python 3.  Adjust the
tests to work under either Python 2 or Python 3.

This introduces a use of b"" string literals, which are supported by
Python 2.7 and Python 3, but not by Python 2.6.

Also, document the required minimum Python version.

DEVELOPER

@@ -5,6 +5,7 @@ How to Build Taskwarrior
       will be utilizing C++11.
     - libuuid
     - gnutls (optional)
+    - python 2.7 or 3 (optional, for running the test suite)
 
   Obtain and build code:
     $ git clone https://git.tasktools.org/scm/tm/task.git task.git