Does not work for UTF-8 string assertions

[darkone23]

Library was failing on my project with an internal error.

https://github.com/pytorch/expecttest/blob/main/expecttest/init.py#L156

Dug into the source code to find the above line with a comment.

Feature should support UTF-8 strings or docs could add a warning saying 'string replacement only supports ascii chars'

[ezyang]

This assert can be relaxed for characters that are valid UTF-8