djblets.cache was using cStringIO to build up data for cache, and

then using the default pickle protocol version to serialize it. This

wasn't compatible on Python 3.
cStringIO is no more, and the equivalent expects Unicode strings and

not bytes, so we now must use io.BytesIO for this purpose. The API is

pretty similar, so it's an easy transition.
pickle's default protocol version on Python 3.x is not the same as it

was in Python 2.x, causing differences in how the data is serialized and

deserialized. We now force the usage of the protocol version defaulted

to on Python 2.x.
A unit test was added for ensuring that Unicode storage still works and

that the serialized data is in the format we expect.

Relevant unit tests pass on Python 2.7, 3.4, 3.5, and 3.6.