drop support for Python < 2.6

[zooko]

Let's drop support for versions of Python < 2.6. According to distrowatch's tables about the versions of packages that come with distros, this would exclude Tahoe-LAFS 1.10 from being usable with the included Python on the following versions of major distros:

distro	excluded version	excluded version release date	oldest supported version of the operating system	oldest supported release date	Python in oldest supported	notes	next version	Python in next version
​Fedora	10	2008-11-25	15	2011-05-24	2.7		rawhide will be 17	2.7.2
​Ubuntu	8.10	2008-10-30	10.04 LTS	2010-04-29	2.6		will be 12.04 precise	2.7.2
​Debian	5.0	2009-02-15	6.0	2011-02-06	2.6		unstable sid will be 7.0	2.7.2
​FreeBSD	7.2	2009-05-04	7.1	2009-01-05	2.5	but upgrading is supposed to be pretty safe within the same major series (7.x) and 7.3 (2010-03-23) has Python 2.6	10 CURRENT	2.7.2
​RedHat	5.7	2011-07-21	5.7	2011-07-21	2.4	I'm told that people tend to install their own newer version of Python when they want to use newer Python programs on older RHEL	(current, not next) 6.2	2.6.6

Among other benefits, dropping support for Pythons older than 2.6 would allow us to remove the dependency on simplejson and the (complex imperatively decided) dependency on pysqlite.

[davidsarah]

I don't understand the "oldest supported" columns for FreeBSD and RedHat, if they're supposed to be referencing the "included" Python.

Another thing that could be dropped is the SHA-256 bindings in pycryptopp, since hashlib has SHA-256 since Python 2.5.