formatter: add argument to change output file of non-plain formatter This allows us to build data not written to the console. That would be doable by ui.pushbuffer()/popbuffer(), but changing the file object seems cleaner.

schemes: move re construction to module-level and python3-ify This makes the schemes extension load correctly in Python 3.

dispatch: cope with sys.version being unicode on Python 3

dispatch: allow testedwith to be bytes or str

ui: fix ui.traceback on Python 3

ui: fix opts labeling on ui.warn et al for Python 3 This is a step towards fixing extension load warnings on Python 3. Note that I suspect there are still some bugs in this area and that things like color won't work, but the code at least executes and prints text to the console correctly now.