this alters and renames the caching function a bit since it caches *to
the group* object but the actual values (in ECDH) come from the testcase
itself

It's more than 60x faster.

* KBKDF: add CounterLocation.MiddleFixed

* tests_kbkdf: add new tests

- Test CounterLocation.MiddleFixed and blocation=

* test_kbkdf_vectors: make use of MIDDLE_FIXED

* docs: document KBKDF CounterLocation.MiddleFixed

* s/blocation/break_location/g

* CHANGELOG: document KBKDF{HMAC,CMAC} CounterLocation.MiddleFixed #7489

* kbkdf: fixed is static across n iterations, only the counter changes

* kbkdf: check break_location is 0..len(fixed)

- Update tests to assert exception is raised when
    break_location < 0 or > len(fixed)

- When asserting for "break_location is ignored when MiddleFixed",
    use break_location=0 instead of =10,
    to ensure we don't raise because of break_location > len(fixed)

- Assert that the right error messages are returned to the user.

* kbkdf: keyword-only break_location

* docs: document kw-only KBKDF{HMAC,CMAC}

As someone who first with Python in 2.4 or so, this habit is going to be hard to break.

* Fix mypy

* Poke for GHA