#279 prevents the crash / incorrect behaviour from #278, but this should be supported -- shouldn't be too difficult but will likely need to reimplement the format string parsing (the stdlib one doesn't notice these escape sequences, probably because they're gone by that point of the parser)

The current behaviours that should be fixed:

1. % format: '\N{snowman} %s' % (a,) should become '\N{snowman} {}'.format(a)
2. f-string: '\N{snowman} {}'.format(a) should become f'\N{snowman} {a}'