Nothing's wrong with them. Though it is more usual to use triple quotes for docstrings even if they fit on one line (see PEP 257, "Triple quotes are used even though the string fits on one line. This makes it easy to later expand it.").

Easy in the sense of more pleasant version control diffs.

Couldn't it cause issues with "object definition" code in REPLs and editors, and interfere with documentation generators, i.e. [Sphinx](http://www.sphinx-doc.org/en/stable/)?

I'd be surprised if so. Most of those tools rely on the built-in parser, which treats all string literals similarly.