MySQLdb's string_literal is a method from the C API and the name is not
natural for python programmers, because it manipulates bytes. It
supports str, unless they have non ASCII characters, so it's better to
always pass bytes.

On python2, this change repairs a regression visible when searching
catalog with translated related keys. This regression was introduced in
the merge of zope4 support, more specifically with 610972af (py3:
Update Shared.DC.ZRDB.{sqltest,sqlvar} monkey patches., 2022-04-16),
because with the new version of the patches this method gets passed
unicode strings, which is fine, unless they contain non ASCII characters.