Hack installation of PyYAML on pypy

Currently PyYAML fails to build with Cython 3.0.0a, thus
it fails to install from sdist using `pip install --pre`.

See yaml/pyyaml#601

.github/workflows/python-package.yml

@@ -26,10 +26,18 @@ jobs:
           ~/.cache/pre-commit
           ~/.cache/pip
         key: ${{ matrix.python_version }}-${{ hashFiles('.pre-commit-config.yaml') }}-${{ hashFiles('setup.py') }}
+
+    - name: Use latest pip
+      run: python -m pip install --upgrade pip
+    - name: Pre-install PyYAML without --pre
+      if: ${{ startsWith(matrix.python_version, 'pypy') }}
+      # Temporary workaround:
+      # PyYAML fails to build with Cython 3.0.0a
+      # see https://github.com/yaml/pyyaml/issues/601
+      run: pip install PyYAML
     - name: Install dependencies
-      run: |
-        python -m pip install --upgrade pip
-        pip install --pre -e '.[dev]'
+      run: pip install --pre -e '.[dev]'
+
     - name: Pre-commit hooks
       if: ${{ matrix.python_version != 'pypy3' && matrix.python_version != '3.6' }}
       run: pre-commit run --all-files