Add config.pickle_protocol, default to version 2

Pickle protocol version 2 has been implicitly used by Pywikibot
since that is the default version for Python 2.  Now that the
package supports Python 3, which defaults to version 3 and has
a version 4 also available, pickling needs to explicitly use
protocol 2 in order that dumps may be accessed by either supported
version of Python.

Change-Id: Ib675990f9accd89b610c2bab1d05e90b29ae4baa
Merge "Add config.pickle_protocol, default to version 2"