py3: for statprof's Chrome output, write json to string, then encode to bytes
`json.dump(obj, fp)` requires `fp.write()` to accept str output, and
since the file pointer we have there only accepts bytes, we need to
change to json.dumps() and then encode as utf-8. We have already done
the same thing for the json (non-Chrome) format in 4b7eb862692e (py3:
encode json output to bytes and use write(), 2018-10-12).
Differential Revision: https://phab.mercurial-scm.org/D6781
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).