ci: use pip constraints for PyPy builds

This feels strictly better. It doesn't fix the busted CI builds. But
the new log output from verbose mode shows it is ignoring our constraints
file and installing a new setuptools version which we're not compatible
with. Fun times.

Author: indygreg

.github/workflows/pypy.yml

@@ -40,7 +40,7 @@ jobs:
 
       - name: Build
         run: |
-          python -m pip wheel -w wheelhouse .
+          python -m pip -v wheel --constraint ci/constraints.txt -w wheelhouse .
 
   test:
     strategy:
@@ -86,7 +86,7 @@ jobs:
 
       - name: Build
         run: |
-          python -m pip install -e .
+          python -m pip -v install --constraint ci/constraints.txt -e .
 
       - name: Test
         run: |