run-tests: make code coverage work on Python 3 This code path was obviously not tested on Python 3 because it blew up in several places due to str/bytes mismatch. For internal code, we normalize paths to bytes. For code calling into `coverage`, we normalize paths to str, which is what `coverage` seems to expect. After this, `run-tests.py -H` works on Python 3! Differential Revision: https://phab.mercurial-scm.org/D7133