Support Python 2.5 again?

[jdavid]

From the work of @jun66j5 it looks like there is some interest to support earlier versions of Python.

Support for Python 2.5 was dropped because it was too hard to make the unit tests working with it, because of the unicode mess.

But now, since the Oid stuff was merged, if I remove the unicode literals imports, there are only four tests failing. Supporting again Python 2.5 looks feasible.

This is related to issue #209