port accumulateMethods and prefixedMethods to Python 3

[exarkun]

The unit tests for SynchronousTestCase depend on these two methods.

[exarkun]

(In [35364]) Branching to 'some-reflect3-5906'

[exarkun]

I filed #5909 for improving the docstrings here.  Build results

[itamar]

Hooray for more test coverage.

1. You should add a note to the test module referencing #5885, for easy grepping of things that need fixing.
2. I named the equivalent util module _utilpy3; either naming scheme is fine, we should just try to be consistent, so either rename yours or if you prefer this scheme I can rename mine.
3. The X Y Z naming scheme is a tiny bit confusing; maybe "Base" and "Sub" and "method"?
4. Needs news file.

Fix the above, then feel free to merge.

[exarkun]

(In [35374]) Merge some-reflect3-5906

Author: exarkun Reviewer: itamar Fixes: #5906

Port a couple twisted.python.reflect methods to Python 3, in support of porting a minimal subset of trial. Also, add some unit tests for these previously untested methods.