Looks good, thank you!
Can you please also add a test for #1514 (comment)? It seems to pass with your change:

        self.assertMarkdownRenders(
            '<span>He replied, "She said \'Hello.\'"</span>',
            '<p><span>He replied, &ldquo;She said &lsquo;Hello.&rsquo;&rdquo;</span></p>'
        )

And please add a changelog entry to docs/changelog.md.

Yes, I thought about adding that test with <span> tags, but none of the existing tests have HTML in the input text, only Markdown. So I instead added a test with parentheses based on facelessuser's comment, which tests a similar situation (closing quotes before non-word character). But I would be happy to add the <span> test as well, if you still prefer. Just let me know.

I think adding the span case is good as well. As noted before, it triggered different logic, and your fix applies to all the broken cases.