CI: remove test against Amaranth 0.3.

Author: jfng

.github/workflows/main.yaml

@@ -13,7 +13,6 @@ jobs:
         - '3.8'
         # this version range needs to be synchronized with the one in pyproject.toml
         amaranth-version:
-        - '0.3'
         - 'git'
       fail-fast: false
     steps:
@@ -27,14 +26,18 @@ jobs:
         if: ${{ matrix.amaranth-version != 'git' }}
         run: |
           pip install 'amaranth==${{ matrix.amaranth-version }}'
-      - name: Downgrade MarkupSafe
-        if: ${{ matrix.amaranth-version == '0.3' }}
-        run: |
-          pip install 'MarkupSafe==2.0.1'
       - name: Install Amaranth from git
         if: ${{ matrix.amaranth-version == 'git' }}
         run: |
           pip install git+https://github.com/amaranth-lang/amaranth.git
       - name: Test
         run: |
           python -m unittest discover
+
+  required: # group all required workflows into one for the required status check
+    needs:
+      - test
+    runs-on: ubuntu-latest
+    steps:
+      - run: |
+          true