gh-145754: Update signature retrieval in unittest.mock to use forwardref annotation format

[mschoettle]

Uses annotation format FORWARDREF instead of the default VALUE when calling inspect.signature.
STRING also seems to work but affects existing tests that check the signature.

• Issue: Autospeccing with unittest.mock is less robust with PEP 649 #145754

Closes #145754

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[JelleZijlstra]

This change looks right to me, please add a unit test and we'll hopefully get it merged!