[1.6.x] Fixed #21355 -- try importing _imaging from PIL namespace first.

    * CPython 2.x may *NOT* have _imaging (broken & needs a error message)

    * CPython 3.x doesn't work

    * PyPy will *NOT* have _imaging (but works?)

    * On some platforms (Homebrew and RHEL6 reported) _imaging isn't available,

      the needed import is from PIL import _imaging (refs #21355)

Restated, that looks like:

        * The only option here is that we're on Python 2.x or PyPy, of which

          we only care about if we're on CPython.

        * If we're on CPython, attempt to ``import _imaging``

        * If we're on CPython, attempt to ``from PIL import _imaging`` and

          ``import _imaging``

            * ``ImportError`` - Bad install, toss an exception

            # We're on CPython (likely 2.x). Since a C compiler is needed to

            # produce a fully-working PIL & will create a ``_imaging`` module,

            # we'll attempt to import it to verify their kit works.

            try:

                from PIL import _imaging as PIL_imaging

            except ImportError:

                try:

                    import _imaging as PIL_imaging

                except ImportError as err: