Martin von Zweigbergk <martinvonz@google.com> [Wed, 17 Jul 2019 12:47:37 -0700] rev 4762
py3: redefine "troublecategories" in evolve as a dict
We'll want to the keys to be bytes and the values to be unicode on
py3. Having it defined as a dict makes that easier (instead of
converting between the types with e.g. pycompat.sysbytes()). It was
kind of ugly to convert between the forms by stripping '_' from the
string anyway.