ci: Install release candidates for 'current release' test workflow (#1996)

* Use release candidates that are on PyPI for verifiying that the public API
  passes tests. This verifies that the release candidates that users are being
  asked to test reflect the release API.
* Use the latest version of pytest.

Author: matthewfeickert

.github/workflows/release_tests.yml

@@ -34,8 +34,8 @@ jobs:
     - name: Install from PyPI
       run: |
         python -m pip install --upgrade pip setuptools wheel
-        python -m pip install pyhf[backends,xmlio]
-        python -m pip install 'pytest~=7.0' pytest-cov
+        python -m pip install --pre pyhf[backends,xmlio]
+        python -m pip install pytest pytest-cov
         python -m pip list
 
     - name: Canary test public API