Evade pypa/pyproject-hooks#74.

Author: Julian

.github/workflows/packaging.yml

@@ -21,6 +21,9 @@ jobs:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies
       run: python -m pip install pep517
+    - name: Evade 'pypa/pep517#74'
+      run: python -m pip install -U setuptools setuptools-scm pip wheel
+      if: startsWith(matrix.python-version, 'pypy')
     - name: Create packages
       run: python -m pep517.build .
     - uses: actions/upload-artifact@master