[3.8] bpo-38270: Fix indentation of test_hmac assertions (GH-17446) (GH-17450)

[3.8] bpo-38270: Fix indentation of test_hmac assertions (GH-17446) (GH-17450)

Since https://github.com/python/cpython/commit/c64a1a61e6fc542cada40eb069a239317e1af36e two assertions were indented and thus ignored when running test_hmac.

This PR fixes it. As the change is quite trivial I didn't add a NEWS entry.

https://bugs.python.org/issue38270
(cherry picked from commit 894331838b256412c95d54051ec46a1cb96f52e7)

Co-authored-by: stratakis <cstratak@redhat.com>
https://bugs.python.org/issue38270

Automerge-Triggered-By: @tiran