Change unicode char to bytes (#5256)

[fabioz]

Just want to check if this runs on ci (if it does I'll go on and fix the pull request with the items below).

• Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst. See changelog/README.rst for details.
• Target the master branch for bug fixes, documentation updates and trivial changes.
• Target the features branch for new features and removals/deprecations.
• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.

Unless your change is trivial or a small documentation fix (e.g., a typo or reword of a small section) please:

• Add yourself to AUTHORS in alphabetical order;

[blueyed]

One failure here: https://travis-ci.org/pytest-dev/pytest/jobs/532281621#L5019

(I wasn't aware that #5256 was about this new code there)
/cc @nicoddemus

[asottile]

this does not do what you want, this will strip backslashes, u, D, 8, and 3 from the right side of the string

[nicoddemus]

Closing in favor of #5271