Handle recursive re-raising of exceptions

[tomage]

Fixes: #318

I implemented what I thought seemed like a fairly sensible way of breaking out of any potential circular path, and also pretty much copied & pasted a unit-test and modified slightly to fit.

I tested this myself on Python 2.7.16 and Python 3.6.6 (I also tested with raise cause from cause on Python 3).

Interestingly, Django has a similar problem, when DEBUG = True. And I just noticed that recently they patched it (see https://code.djangoproject.com/ticket/29393 and https://github.com/django/django/blob/400ec5125ec32e3b18d267bbb4f3aab09d741ce4/django/views/debug.py#L401).

[tomage]

Hm.. Now that I look more closely at the Django code, I should probably copy their approach to the test, and use threads to ensure this won't stall tests in case the infinite-loop mechanism breaks (see https://github.com/django/django/pull/10730/files#diff-7ac36566c31afe0c5ce04aaecc0dd1dbR425).
I'll amend tomorrow.