[toward-py3k] rewrite to "except AnException as exc:" (part of #2711624) The older form is not necessary since we dropped python 2.5 compability. This will help Python 3.3 experimentation.

[toward-py3k] remove import with_statement (part of #2711624) They are unneeded since we dropped python 2.5 compatibility. This will help Python 3.3 experimentation.

pkg: drop python 2.5 compatibility (closes #2711624) 2.5 is not common anymore. Dropping compatibility will help moving to a code base compatible with Python 3.3