Add Python 3.10a4 Support

[crwilcox]

This fixes #949 by adding the alpha candidate for 3.10, along with necessary keys.

[crwilcox]

@mayeut do you have an opinion on this? I think it would be useful to be able to use the docker images to build against the upcoming release (https://www.python.org/downloads/release/python-3100a4/)

Also note, alpha 5 is expected soon so I can rev this before we merge also.

[mayeut]

@crwilcox,
We usually wait for a beta release for new python because the C API/ABI is allowed to change in alpha stage which might lead to all sorts of bugs if someone deploys something built with an alpha version on PyPI.

[crwilcox]

@mayeut that is reasonable. If this causes burden to keep up to date I can definitely understand, though I would find value in this since I am hoping to flight these builds and hopefully let CPython dev know if something goes awry. That said, waiting until beta isn't a huge deal :)