Pep 572 update (python#654)

- Remove changes to comprehension scope
- Make := in comprehensions assign to containing non-comprehension scope
- Clarify binding precedence (tighter than comma, not at same level as =)
- Remove mention of more complex targets in the future
- Explicitly disallow toplevel :=
- Rewrite section on differences with =, enumerating all of them
- Remove "here's how this could be written without :=" from examples

- Tweak first paragraph of "Syntax and semantics"
- Add "Exception cases" (various explicit prohibitions)
- Clarify that lambda is a containing scope
- Clarify that := and = just don't mix
- Added "Open questions" section
- Added two new rejected alternatives: "Allowing commas to the right"
  and "Always requiring parentheses"
- Minor edits

* Start a section on real code

* Correct/clarify "commas to the right" section

* Add Tim and Guido as authors

* Update abstract to mention :=

* Rule out targets conflicting with comprehension loop control

* Add timcode.txt as Appendix A

* Add os.fork() example

* Add TODOs about evaluation order