bpo-1635741: Port _sha3 module to multi-phase init (GH-21855)

Port the _sha3 extension module to multi-phase init (PEP 489).
Convert static types to heap types.

Author: koubaa

Misc/NEWS.d/next/Core and Builtins/2020-08-13-07-19-21.bpo-1653741.fubBkb.rst

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+Port :mod:`_sha3` to multi-phase init.  Convert static types to heap types.