Reasoning being that this make it easier to catch pypy3
regressions as they happen.

* Fixed some very simple pypy3 failures (largely to do with
  different exception strings)
* Splits pypy3_bugs.txt into three files
  - one for bugs that cause hard crashes (which we don't want to
    run in Travis at all);
  - one for bugs that are probably unfixable because they're just
    due to implementation details (e.g. when destructors are
    called).
  - all other bugs remain in pypy3_bugs.txt
  (The categorization has been done fairly quickly, so some bugs
  may be in the wrong place)
* Made sure (hopefully) all bugs are now categorized, so a basic
  runtests.py with pypy3 should hopefully pass
* Changed pypy3 to be required in Travis
* Added an extra (optional) test that runs through pypy3_bugs.txt.
  The majority of this is expected to fail. This requires an
  extra option to runtest.py "--listfile", which just runs through
  the tests listed in the file.

I haven't made pypy2 a required test in this commit - since Python2 support is deprecated soon, there seemed limited value in putting much effort into pypy2.

Added faulthandler to runtests in the hope of being able to pin-down segmentation faults better on Travis

FileListExcluder matches regexes, not just name. Uses the same mechanism as is used for processing string passed on commandline