[Biopython] Ending Python 2.6 support after Biopython 1.68

[Peter Cock]

Dear Biopythoneers,

Both Biopython 1.66 (21 October 2015) and today's release of
Biopython 1.67 (8 June 2016) will work on Python 2.6 but give
a warning at install time that support for this is deprecated.

In line with our deprecation policy, unless there are multiple
objections with strong justifications, I expect our next release
(Biopython 1.68) will be our final release to support Python 2.6,

http://biopython.org/wiki/Deprecation_policy

For reference, while the current NumPy 1.11.x releases still support
Python 2.6, NumPy 1.12.0 will drop Python 2.6 support (targeting
Python 2.7, and 3.4 onwards). Most other major Python libraries
have already or are in the process of dropping Python 2.6 support.

Thanks,

Peter