Pin flake8<5 in workflow.

Author: erykoff

.github/workflows/build.yaml

@@ -34,7 +34,7 @@ jobs:
         shell: bash -l {0}
         run: |
           mamba install -y -q \
-            flake8 \
+            "flake8<5" \
             pytest pytest-flake8 pytest-xdist pytest-openfiles pytest-cov pytest-session2file
 
       - name: List installed packages