gh-141907: Better handle support for SHA3 for test_hashlib (GH-141908)

* test_hashlib: better handle support for SHA3

It's possible that the SSL library supports only SHA3 algo and doesn't
have SHAKE one.

The current test wrongly detect this and set both HASH and HASHXOF to
None expecting to have the extra SHA3 attributes present but this should
only be true for SHAKE algo.

To better handle this, move the HASH condition to a dedicated try-expect
condition and check if HASHXOF is None in the relevant code effectively
checking if SHA3 is supported by the SSL library but SHAKE algo needs to
use the sha3module one.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

* rework the conditional import for all its attrs

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Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>