Port twisted.python.failure to Python 3

[exarkun]

Many parts of Twisted depend on twisted.python.failure. Perhaps the debug functionality can be skipped for the initial pass, but most or all of the rest of the functionality will probably be required fairly soon.

This is a dependency for porting even a minimal subset of trial (#5874).

Some parts of the #5802 branch may be useful in this effort.

[exarkun]

This depends on #5913 and

[exarkun]

... #5932.

[itamarst]

(In [35528]) Branching to 'failure-python3-5931'

[itamarst]

(In [35543]) Branching to 'failure-python3-5931-2'

[itamar]

Ready for review; running tests at  http://buildbot.twistedmatrix.com/boxes-supported?branch=/branches/failure-python3-5931-2

Some notes:

1. I punted on a couple of the really hideous failing tests for _findFailure, and opened separate ticket referenced in the code. Didn't seem important enough to block progress.
2. Generator tests are effectively unchanged, just moved back into main test module; I only changed g.next() to next(g) and suppressed one test on Python 3.

[exarkun]

Thanks. Surprisingly unterrible, really.

1. We probably should already have started a guide for application developers, enumerating the changes they might need to make (like different exception handling around Failure.trap calls). Can you start one for this ticket and put this item in it? Then I guess we should go back and add stuff to it for all the tickets we resolved already.
2. twisted/test/test_failure.py, On both Python 2 and Python 3, the underlying exceptio is passed on: - missing an n
3. twisted/test/test_failure.py needs a lot of help overall :/ Can you file tickets for:
   1. Improving the docstring for test_notTrapped
   2. Splitting up test_printingSmokeTest into separate tests for each method it is calling, as well as adding some more serious assertions
   3. The same for test_printingCapturedVarsSmokeTest
   4. Making failure.startDebugMode work in a less bad way. We should do something that doesn't involve monkey-patching Failure. (See next point)
4. The change to TestDebugMode.setUp is wrong for Python 2. Failure.__dict__['__init__'] is the actual function object, Failure.__init__ is an unbound method. With the change, failure.Failure.__init__ = origInit now puts an unbound method into Failure.__dict__.
5. Seems like the inlineCallbacks tests are in the wrong place. They'd make more sense in a Deferred-related test module (in particular, not importing twisted.internet to test twisted.python would make a lot of sense). Not sure this is really actionable here, but maybe there's another ticket to be filed for it.

[itamarst]

(In [35547]) Address review comments 2 and 4. Refs #5931

[itamar]

Opened #5952, #5953, #5954 and #5955 to address review comments 3 and 5.

[itamarst]

(In [35548]) Howto covering python 3 porting, starting with failure. Refs #5931

[itamar]

OK, addressed review comments.

[itamar]

And a new build started:  http://buildbot.twistedmatrix.com/boxes-supported?branch=/branches/failure-python3-5931-2

[exarkun]

Looks good, please merge.

[itamarst]

(In [35549]) Merge failure-python3-5931-2

Author: itamar, vperic Review: exarkun Fixes: #5931

Port twisted.python.failure to Python 3, and add new porting howto.