Remove twisted.python.threadable's dependency on twisted.python.hook

[itamar]

threadable.synchronize is the only code in Twisted that depends on hook. Rewriting it to not use hook would mean less code to port to Python 3.

[exarkun]

(In [35477]) Branching to 'unhook-threadable-5860'

[exarkun]

(In [35478]) Re-implement threadable.synchronize with a function wrapper instead of a hook

refs #5860

[exarkun]

Removed the dependency and ported threadable to Python 3. The astute reader will notice that synchronize is a class decorator.

 Build results

[itamar]

1. You can't raise unittest.FailTest in Python 3. Since this only happens if the test fails, it just means the failure (which doesn't happen since the code is correct) is obscured. #5885 will fix this, so I don't think this worth fixing.
2. Not depending on reactor is good.
3. I opened a ticket to improve testing of the synchronize decorator (#5938).

I note that Twisted cannot run without threading being available (since twisted.python.context uses it for example), so perhaps another ticket should be filed for simplifying the module. Not that we'll ever get around to it.

So, looks good. I will merge.

[itamarst]

(In [35498]) Merge unhook-threadable-5860

Author: exarkun Review: itamar Fixes: #5860

Port twisted.python.threadable to Python 3.