CI: Test without pre-installing from requirements.txt

Author: effigies

.github/workflows/test.yml

@@ -25,9 +25,7 @@ jobs:
     strategy:
       matrix:
         python-version: ["2.7", "3.6", "3.7", "3.8", "3.9", "3.10", "3.11"]
-        requires: ["requirements.txt"]
-        # Can't drop requirements.txt without build isolation
-        # requires: ["", "requirements.txt"]
+        requires: ["", "requirements.txt"]
 
     steps:
       - name: Checkout repo