sslutil: move and document verify_mode assignment Consolidating all the SSLContext options setting makes the code a bit easier to read.

tests: use --insecure instead of web.cacerts=! --insecure is the proper and documented way to do this. The end result is the same: dispatch will set web.cacerts to ! when --insecure is passed. This patch is necessary to refactor handling of web.cacerts in upcoming patches.

help: remove references to "Python 2.6 or later" We require Python 2.6. So there is no value to these docs.