Fix path to requirements file in mypy CI job

Author: mx-moth

.github/workflows/ci.yaml

@@ -135,7 +135,7 @@ jobs:
         uses: actions/cache@v3
         with:
           path: '.mypy_cache'
-          key: mypy-${{ runner.os }}-py${{ env.python-version }}-${{ hashFiles('requirements.txt') }}
+          key: mypy-${{ runner.os }}-py${{ env.python-version }}-${{ hashFiles(format('continuous-integration/requirements-{0}.txt', env.python-version)) }}
 
       - run: flake8 src/ tests/
       - run: isort --diff --check-only src/ tests/