Support return type shorthand for D401

[ooliver1]

I cannot find the name of this, if it is part of the numpy style, parsed from ext.napoleon, ext.autodoc, Sphinx or part of ReSructuredText itself, but an example is in the ext.napoleon example

    @property
    def readonly_property(self):
        """str: Properties should be documented in their getter method."""
        return "readonly_property"

    @property
    def readwrite_property(self):
        """:obj:`list` of :obj:`str`: Properties with both a getter and setter
        should only be documented in their getter method.

        If the setter method contains notable behavior, it should be
        mentioned here.
        """
        return ["readwrite_property"]

In general:

def func(...) -> T:
    """T: Summary"""

Currently, D401 ("First line of docstring should be in imperative mood:") does not detect non-imperative in the summary as it sees the first word as str, :class:`str`, etc.

[charliermarsh]

Converting to an issue, seems reasonable to support!