gh-136912: fix handling of `OverflowError` in `hmac.digest` (#136917)

The OpenSSL and HACL* implementations of HMAC single-shot
digest computation reject keys whose length exceeds `INT_MAX`
and `UINT32_MAX` respectively. The OpenSSL implementation
also rejects messages whose length exceed `INT_MAX`.

Using such keys in `hmac.digest` previously raised an `OverflowError`
which was propagated to the caller. This commit mitigates this case by
making `hmac.digest` fall back to HMAC's pure Python implementation
which accepts arbitrary large keys or messages.

This change only affects the top-level entrypoint `hmac.digest`, leaving
`_hashopenssl.hmac_digest` and `_hmac.compute_digest` untouched.