py3: convert key to str to make kwargs.pop work in mq
The keys are passed here and there as unicodes and our transformer make things
bytes. Due to that, mq was not poped and this results in error on Py3.
Here we abuse r'' to make that str on Python 3.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.