ch-image --help crashes with LC_ALL=C under Python 3.6

[reidpr]

For example:

$ python3 --version
Python 3.6.8
$ LC_ALL=C bin/ch-image --help
Traceback (most recent call last):
  [...]
  File "/usr/lib64/python3.6/argparse.py", line 2372, in _print_message
    file.write(message)
UnicodeEncodeError: 'ascii' codec can't encode character '\u201c' in position 1792: ordinal not in range(128)

This was printed inside argparse. Messages we log don't crash but get escaped in an ugly way:

$ LC_ALL=en_US.UTF-8 bin/ch-image list
crash “me”
$ LC_ALL=C bin/ch-image list
crash \u201cme\u201d

It doesn't seem to happen under Python 3.9. I haven't tried to narrow down the version boundary.