What's the difference on docstrings with triple SINGLE quotes and triple DOUBLE quotes?

No. They are the same. The only difference is that the first one can contain a sequence of three unescaped double quotes, while the second can contain a sequence of three unescaped single quotes. (In other words, because the delimiters are different, there is a slight difference in what characters you can use inside them.)

Docstrings are just regular strings, and in Python there is no difference between the different string delimiters, except that, of course, you can't use the string delimiter inside the string.

The informational document PEP 257 -- Docstring Conventions recommends to use """triple double-quotes""" for consistency, and all their examples show the same:

For consistency, always use """triple double quotes""" around docstrings. Use r"""raw triple double quotes""" if you use any backslashes in your docstrings. For Unicode docstrings, use u"""Unicode triple-quoted strings""".

Whether to use single quotes or double quotes is only a stylistic issue in practice. There will be no difference in formatting when generating and publishing docs from these strings.

Choose whatever style you want. Personally I use single quotes everywhere I can in Python.

The documentation states:

"String literals can be enclosed in matching single quotes (') or double quotes (")."

It doesn't matter which one you decide to use. What does matter, is that you stick with your decision. It is good practice to choose a style and stick with it.