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Adding some tests to non-default code paths in hmac.py to get to 100% coverage.

Thanks for the patch. Could you rewrite it using self.assert* methods instead of asserts?
I’m not an hmac expert, so you may have to wait a bit for another core developer to see and commit the patch.

Updated to use the correct assert* methods.

Some style comments:
- don't use "except: self.fail()", just let the exception pass through
- when monkeypatching hmac.new, use a try...finally block so that the mock doesn't stay in place if the test fails for whatever reason
- it's a bit of a nit, but when using "with warnings.catch_warnings", I would put the asserts on the context manager result outside of the "with" block
- I get a warning in test_noncallable_digestmod, it would be nice to silence it:
/home/antoine/cpython/default/Lib/test/test_hmac.py:254: RuntimeWarning: No block_size attribute on given digest object; Assuming 64.
 h = hmac.HMAC(b"key", b"", hmac)

IIRC we also need a signed contributor agreement in order to commit your patch.

If there isn't a signed contributor agreement I'll put up a new version of the patch.
In msg156758 Antoine said 'don't use "except: self.fail()", just let the exception pass through'. There are several of these in the existing code. Should they all be removed or must it be done on a case by case basis?

The patch is out of date and needs to be refreshed.

The only things I think we should add to the current hmac tests are test_update_error_handling and test_with_invalid_msg. 
For test_withnoncallable_digestmod(), hmac itself seems it can no longer be used:
>>> hmac.HMAC(b"gggg", None, "hmac")
<module '_hashlib' from '/Users/arjun/Python/Sources/cpython/build/lib.macosx-11.4-x86_64-3.11-pydebug/_hashlib.cpython-311d-darwin.so'>
using new
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/Users/arjun/Python/Sources/cpython/Lib/hmac.py", line 61, in __init__
 self._init_hmac(key, msg, digestmod)
 File "/Users/arjun/Python/Sources/cpython/Lib/hmac.py", line 69, in _init_hmac
 self._hmac = _hashopenssl.hmac_new(key, msg, digestmod=digestmod)
ValueError: unsupported hash type hmac
For tests test_small_block_size and test_no_block_size, a custom .blocksize cannot be set (changing .block_size has no difference on digest()):
>>> hmac.HMAC(b"gggg", None, "md5").blocksize = 15
<module '_hashlib' from '/Users/arjun/Python/Sources/cpython/build/lib.macosx-11.4-x86_64-3.11-pydebug/_hashlib.cpython-311d-darwin.so'>
using new
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
AttributeError: 'HMAC' object attribute 'blocksize' is read-only
```
class myHMAC(hmac.HMAC):
 blocksize = 1
 def __init__(self, key, msg, digestmod_):
 super().__init__(key, msg, digestmod=digestmod_)
h = myHMAC(b"key", b"", digestmod_="md5")
h.digest() # <- works perfectly fine.
```
Does this sound okay? I can go ahead an implement it if so.

would the update invalid test belong in the TestVectorsTestCase class or should I make a new one?