Constrain allowed hash algos to "better than sha256" of PEP 427

[MrMino]

This has surfaced within #3.

PEP 427 declares the following requirement on the RECORD file:

The hash algorithm must be sha256 or better; specifically, md5 and sha1 are not permitted, as signed wheel files rely on the strong hashes in RECORD to validate the integrity of the archive.

It isn't rigorously obvious what does the "or better" mean in this context.

Request clarification on this & when a response is received - improve the WheelRecord class accordingly.

[e2thenegpii]

https://mail.python.org/archives/list/python-dev@python.org/thread/TP5DEDAW5TUQS44BY6RE2HHRRGOL56HZ/

[MrMino]

Good, that solves that part then. Looks like only SHA-256, SHA-384, and SHA-512 are permitted? That's a bit weird way to state it...