Allow blank line between docstring and coroutine definition

[rytilahti]

#426 fixed D202: No blank lines allowed after function docstring errors for blackified inner functions, but it seems this was only a partial fix and D202 is still being reported for async defed inner functions.

Example code based on #361 (comment):

"""foo."""

def spam():
    """Spam."""

    def ham():
        return "ham"

    return "spam"


def async_spam():
    """Async spam."""

    async def ham():
        return "ham"

    return "spam"

produces the following:

/tmp/test.py:13 in public function `async_spam`:
        D202: No blank lines allowed after function docstring (found 1)

[sambhav]

This is fixed now.

[sambhav]

Related changes #467