CI: Refactor workflows, update test matrix

* Split test, integration-test, and release workflows,
  simplifying config and logic for each.
* Use environment to separate secrets for release workflow.
* Update test matrix with newer Django and Python versions.

.github/workflows/integration-test.yml

@@ -0,0 +1,88 @@
+name: integration-test
+
+on:
+  pull_request:
+  push:
+    branches: [ "main", "v[0-9]*" ]
+    tags: [ "v[0-9]*" ]
+  workflow_dispatch:
+  schedule:
+    # Weekly test (on branch main) every Thursday at 12:15 UTC.
+    # (Used to monitor compatibility with ESP API changes.)
+    - cron: "15 12 * * 4"
+
+
+jobs:
+  skip_duplicate_runs:
+    # Avoid running the live integration tests twice on the same code
+    # (to conserve limited sending quotas in the live ESP test accounts)
+    runs-on: ubuntu-20.04
+    outputs:
+      should_skip: ${{ steps.skip_check.outputs.should_skip }}
+    steps:
+      - id: skip_check
+        # uses: fkirc/skip-duplicate-actions@v3.4.0
+        uses: fkirc/skip-duplicate-actions@4c656bbdb6906310fa6213604828008bc28fe55d
+        with:
+          concurrent_skipping: "same_content"
+          skip_after_successful_duplicate: "true"
+          cancel_others: "true"
+          do_not_skip: '["pull_request", "workflow_dispatch", "schedule"]'
+
+  test:
+    name: ${{ matrix.config.tox }} ${{ matrix.config.options }}
+    runs-on: ubuntu-20.04
+    needs: skip_duplicate_runs
+    if: needs.skip_duplicate_runs.outputs.should_skip != 'true'
+    timeout-minutes: 15
+    strategy:
+      fail-fast: false
+      matrix:
+        # Live API integration tests are run on only one representative Python/Django version
+        # combination, to avoid rapidly consuming the testing accounts' entire send allotments.
+        config:
+          - { tox: django40-py310-amazon_ses, python: "3.10" }
+          - { tox: django40-py310-mailgun, python: "3.10" }
+          - { tox: django40-py310-mailjet, python: "3.10" }
+          - { tox: django40-py310-mandrill, python: "3.10" }
+          - { tox: django40-py310-postal, python: "3.10" }
+          - { tox: django40-py310-postmark, python: "3.10" }
+          - { tox: django40-py310-sendgrid, python: "3.10" }
+          - { tox: django40-py310-sendinblue, python: "3.10" }
+          - { tox: django40-py310-sparkpost, python: "3.10" }
+
+    steps:
+      - name: Get code
+        uses: actions/checkout@v2
+      - name: Setup Python ${{ matrix.config.python }}
+        uses: actions/setup-python@v2
+        with:
+          python-version: ${{ matrix.config.python }}
+      - name: Install tox
+        run: |
+          set -x
+          python --version
+          pip install tox
+          tox --version
+      - name: Test ${{ matrix.config.tox }}
+        run: |
+          tox -e ${{ matrix.config.tox }}
+        continue-on-error: ${{ contains( matrix.config.options, 'allow-failures' ) }}
+        env:
+          CONTINUOUS_INTEGRATION: true
+          TOX_FORCE_IGNORE_OUTCOME: false
+          ANYMAIL_RUN_LIVE_TESTS: true
+          ANYMAIL_TEST_AMAZON_SES_ACCESS_KEY_ID: ${{ secrets.ANYMAIL_TEST_AMAZON_SES_ACCESS_KEY_ID }}
+          ANYMAIL_TEST_AMAZON_SES_REGION_NAME: ${{ secrets.ANYMAIL_TEST_AMAZON_SES_REGION_NAME }}
+          ANYMAIL_TEST_AMAZON_SES_SECRET_ACCESS_KEY: ${{ secrets.ANYMAIL_TEST_AMAZON_SES_SECRET_ACCESS_KEY }}
+          ANYMAIL_TEST_MAILGUN_API_KEY: ${{ secrets.ANYMAIL_TEST_MAILGUN_API_KEY }}
+          ANYMAIL_TEST_MAILGUN_DOMAIN: ${{ secrets.ANYMAIL_TEST_MAILGUN_DOMAIN }}
+          ANYMAIL_TEST_MAILJET_API_KEY: ${{ secrets.ANYMAIL_TEST_MAILJET_API_KEY }}
+          ANYMAIL_TEST_MAILJET_SECRET_KEY: ${{ secrets.ANYMAIL_TEST_MAILJET_SECRET_KEY }}
+          ANYMAIL_TEST_MANDRILL_API_KEY: ${{ secrets.ANYMAIL_TEST_MANDRILL_API_KEY }}
+          ANYMAIL_TEST_POSTMARK_SERVER_TOKEN: ${{ secrets.ANYMAIL_TEST_POSTMARK_SERVER_TOKEN }}
+          ANYMAIL_TEST_POSTMARK_TEMPLATE_ID: ${{ secrets.ANYMAIL_TEST_POSTMARK_TEMPLATE_ID }}
+          ANYMAIL_TEST_SENDGRID_API_KEY: ${{ secrets.ANYMAIL_TEST_SENDGRID_API_KEY }}
+          ANYMAIL_TEST_SENDGRID_TEMPLATE_ID: ${{ secrets.ANYMAIL_TEST_SENDGRID_TEMPLATE_ID }}
+          ANYMAIL_TEST_SENDINBLUE_API_KEY: ${{ secrets.ANYMAIL_TEST_SENDINBLUE_API_KEY }}
+          ANYMAIL_TEST_SPARKPOST_API_KEY: ${{ secrets.ANYMAIL_TEST_SPARKPOST_API_KEY }}