[Biopython] Preparing to drop Python 3.3 support

[Peter Cock]

Dear Biopythoneers,

Do any of you still use Python 3.3?

Biopython currently supports and is regularly tested on Python 3.3, 3.4,
3.5 (the current latest release) and also Python 2.6 and 2.7 as well.

(I'm talking about the main C version of Python from python.org here,
we also test under PyPy and Jython).

As of Biopython 1.66, we have deprecated support for Python 2.6.
Based on http://biopython.org/wiki/Deprecation_policy we'll still
support Python 2.6 for Biopython 1.67 and in principle any other
releases we make up to October 2016.

For the next release, Biopython 1.67, I would like to deprecate support
for Python 3.3. This has various codec fixes which will let us remove
some workarounds in our code, and comes with pip which will make
installation simpler in the long run (Python 2.7.9 onwards also has
the ensurepip module).

Phasing out support for Python 2.6 and 3.3 is in line with other Scientific
Python packages like matplotlib, Bokeh, pandas, and (or more direct
relevance), numpy:

https://mail.scipy.org/pipermail/numpy-discussion/2015-December/074331.html

Regards,

Peter