Ensure that HMAC objects are properly initialized

[picnixz]

Bug report

Bug description:

This affects the following situations:

• _hmac.new() frees an uninitialized HACL pointer when the key exceeds UINT32_MAX
• _hmac_HMAC_copy_impl runs HASHLIB_INIT_MUTEX(copy) after the fallible hmac_copy_state

The solution is simply to always entirely initialize the HMAC object before working with it. We also change the usage of PyObject_New to tp->tp_alloc directly so that we only alter fields for which the 0-value would not be suitable.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-155843: properly initialize HMAC objects to prevent crashes after allocation failures #155845
• [3.15] gh-155843: properly initialize HMAC objects to prevent crashes after allocation failures (GH-155845) #157036
• [3.14] gh-155843: properly initialize HMAC objects to prevent crashes after allocation failures (GH-155845) #157037