Better del support

[jayvdb]

Detect Python 3.2 and lower SyntaxError for deleting
variable referenced in nested scope.

[jayvdb]

Still needs a bit of polish, but I'd like confirmation that I am not wasting my time before proceeding with the current strategy.

A real world case is https://hg.python.org/cpython/file/tip/Lib/opcode.py

This would likely solve the original bug report in https://bugs.launchpad.net/pyflakes/+bug/1308508 .

(And https://bugs.launchpad.net/pyflakes/+bug/1461208 should be looked at as part of this)

The DoctestScope could be pulled out to be a separate changeset (or an alternative workaround found). There are related bugs which could be solved as part of this; e.g. https://bugs.launchpad.net/pyflakes/+bug/1178807

I need to see whether the assignment checks can also be un-deferred, as that would reduce the amount of code restructure required to support del properly.

[sigmavirus24]

if node is not None?

[bitglue]

Does this need a refresh after those other two PRs were split off?

[jayvdb]

It certainly will need to be freshened, and a bit of polish.