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	oldest supported release date	Python in oldest supported	notes
​Fedora	10	2008-11-25	15	2011-05-24	2.7
​Ubuntu	8.10	2008-10-30	10.04 LTS	2010-04-29	2.6
​Debian	5.0	2009-02-15	6.0	2011-02-06	2.6
​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
​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

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.