since python/typeshed#5784

since at least here: python/typeshed#5649

Two fixups to keep mypy happy.

The first handles that `get_notAfter` could return None. Before this
would raise a generic `AttributeError`; now it raises a ValueError with
specific message.

The second ensures the callback to `set_verify` returns bool, not
NoneType. (AFAICS this was fine because PyOpenSSL only ever used the
truthiness of the callback's return value.)

Had to make some more invasive changes to our media code. I replaced our
naughty calls to `_getdecoder` with calls to [check_codec], introduced
about seven years ago.

[check_codec]: python-pillow/Pillow@799e831

mypy didn't like our use of `_getexif`. The [Pillow 3.1.0 release
notes][pillow-3.1] describe this as

> private, experimental, but generally widely used

There's a public `getexif()` without an underscore, but that's only
available since [Pillow 6.0.0][pillow-6]

[pillow-3.1]: https://pillow.readthedocs.io/en/stable/releasenotes/3.1.0.html#jpegimageplugin-getexif
[pillow-6]: https://pillow.readthedocs.io/en/stable/releasenotes/6.0.0.html#added-exif-class

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

I'm not sure why newer mypy requires these---perhaps it's a consequence
of pulling in a newer mypy-zope?

I think struct_rusage has existed for a couple of decades now, see https://github.com/python/cpython/blame/bb3e0c240bc60fe08d332ff5955d54197f79751c/Modules/resource.c#L32