Cache pip per python version

.github/workflows/tests.yml

@@ -33,9 +33,9 @@ jobs:
         if: startsWith(runner.os, 'Linux')
         with:
           path: ~/.cache/pip
-          key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}
+          key: ${{ runner.os }}-pip-${{ matrix.python }}-${{ hashFiles('**/requirements.txt') }}
           restore-keys: |
-            ${{ runner.os }}-pip-
+            ${{ runner.os }}-pip-${{ matrix.python }}
 
       # Cache yarn
       - name: Get yarn cache directory path