Fix script for PyPI preparation

[lebigot]

As of version https://github.com/lebigot/uncertainties/tree/59998e1760a8d3e18029fb1dde8d16e5757eaca9, the PyPI preparation script is broken: this is what continuous integration shows, as well as pip install in Python 2.7, when doing import uncertainties.

[benabel]

Thé script should not be necessary when #110 will be merged. Only master branch will be necessary ans python 2 and python 3 should be published from master.

[lebigot]

The Python 2.3/2.7/3 handling mechanism is indeed not needed anymore, with the single Python 2.7/Python 3 source.

The script is still needed however for packaging from a stable release branch (and not directly from master).