make from_bytes() accept python's buffer API

[arvidn]

otherwise it's not possible to directly pass in a slice of a bytes object. i.e. .from_bytes(b[1:]). It would have to be copied into a new bytes object first, i.e. .from_bytes(bytes(b[1:]))

[coveralls-official]

Pull Request Test Coverage Report for Build 6685348061

• 8 of 8 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.008%) to 86.554%

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Totals
Change from base Build 6684093624:	0.008%
Covered Lines:	9739
Relevant Lines:	11252

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💛 - Coveralls