Revise PEP 453 to be docs-only for 2.7 & 3.3

- all functional changes are now 3.4 only
- still proposes docs changes for 2.7 & 3.3
- notes current effort to create a Windows installer for pip
- notes possibility of a future PEP to provide a combined
  CPython 2.7, pip and Python Launcher installer from python.org
Additional PEP 453 fixes