fix: run requirements cron job on even weeks only

.github/workflows/requirements-cron.yml

@@ -2,12 +2,28 @@ name: Requirements (scheduled)
 
 on:
   schedule:
-    - cron: "0 2 */14 * *"
+    - cron: "0 2 * * 1"
   workflow_dispatch:
 
 jobs:
+  is-even-week:
+    name: Check week number is even
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Determine if week number is even
+        run: |
+          WEEK_NUMBER=$(date +%U)
+          IS_EVEN_WEEK="$(($WEEK_NUMBER % 2 == 0))"
+          echo "This is week $WEEK_NUMBER. It even week: $IS_EVEN_WEEK"
+          echo ::set-output name=is-even-week::"$IS_EVEN_WEEK"
+        id: is-even-week
+    outputs:
+      is-even-week: ${{ steps.is-even-week.outputs.is-even-week }}
+
   pip-constraints:
     name: Update pip constraints files
+    needs: is-even-week
+    if: needs.is-even-week.outputs.is-even-week != 0
     runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
@@ -26,6 +42,8 @@ jobs:
 
   pre-commit:
     name: pre-commit autoupdate
+    needs: is-even-week
+    if: needs.is-even-week.outputs.is-even-week != 0
     runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2