Single Python 2+3 code. Universal wheel.

There is now a single code base for Python 2 and Python 3.

Python 2.7 is now the minimal Python version.

The installation can also now be done through a universal wheel, which helps with some external projects (see issue #106, for instance).

This Git version should have the same uncertainties code as PyPI version 3.1.4, but some accompanying files have been updated. This should have no impact on users. Developers are better off using this Git 3.1.4 version.