Allow import failure for internal _hashlib module

[WillChilds-Klein]

Issue #, if available:

n/a

Description of changes:

This change allows the import of _hashlib to fail. _hashlib is a python-internal module (i.e. it is not meant to be used by applications). It is not present in some build configurations of CPython 3.9, so without this PR the sagemaker SDK cannot be loaded in those runtimes. _hashlib.HASH is only used for type annotations in this library, so conditionally relaxing type constraints on import failure should have no bearing on runtime behavior.

Testing done:

• internal testing in a brazil workspace, please ping me for details
• existing GitHub CI

Merge Checklist

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Tests

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