Make it more obvious that buildbot requires python2.6

[johnkw]

easy_install buildbot fails with gcc header errors with python on Cent 5, but python2.6 appears to have worked.

Would be nice if the "egg" could present a helpful dependency failure or something.

Also, mentioning that version in the docs would be good.

[dustin]

It requires 2.5, as the requirements docs say:

http://buildbot.net/buildbot/docs/current/manual/installation.html#requirements

though slaves should run on 2.4, according to those same docs. So, if you have a slave that won't run on 2.4, please file a new bug.

[johnkw]

Upon further investigation, the issue of bad dependency checks actually lies in the pysqlite project.