limit greenlet dependency to pypi-listed platforms

The setup requirements have been modified such ``greenlet`` is a default
requirement only for those platforms that are well known for ``greenlet``
to be installable and for which there is already a pre-built binary on
pypi; the current list is ``x86_64 aarch64 ppc64le amd64 win32``. For other
platforms, greenlet will not install by default, which should enable
installation and test suite running of SQLAlchemy 1.4 on platforms that
don't support ``greenlet``, excluding any asyncio features. In order to
install with the ``greenlet`` dependency included on a machine architecture
outside of the above list, the ``[asyncio]`` extra may be included by
running ``pip install sqlalchemy[asyncio]`` which will then attempt to
install ``greenlet``.

Additionally, the test suite has been repaired so that tests can complete
fully when greenlet is not installed, with appropriate skips for
asyncio-related tests.

Fixes: #6136
Change-Id: I8f3a1c00a4a8b6a273484af1da1f7aaadf588ae7

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