temp: use python 3.8 when upgrading requirements

until we're ready to upgrade to 3.11 or 3.12

cf openedx/.github#131

.github/workflows/upgrade-python-requirements.yml

@@ -13,6 +13,7 @@ jobs:
    call-upgrade-python-requirements-workflow:
     with:
        branch: ${{ github.event.inputs.branch }}
+       python_version: "3.8"
        # optional parameters below; fill in if you'd like github or email notifications
        # user_reviewers: ""
        # team_reviewers: ""
@@ -22,3 +23,4 @@ jobs:
        edx_smtp_username: ${{ secrets.EDX_SMTP_USERNAME }}
        edx_smtp_password: ${{ secrets.EDX_SMTP_PASSWORD }}
     uses: openedx/.github/.github/workflows/upgrade-python-requirements.yml@master
+