Add markdown-link-check github action.

Replaces CLI action run through tox.
Closes Python-Markdown#1243.

.github/workflows/process.yml

@@ -22,3 +22,14 @@ jobs:
           change log describing the changes you have made. Doing so will help to ensure your contribution is accepted.
 
           Please see the [Contributing Guide](https://python-markdown.github.io/contributing/#pull-requests) for details.
+
+  markdown-link-check:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: gaurav-nelson/github-action-markdown-link-check@v1
+      with:
+        use-quiet-mode: yes
+        use-verbose-mode: yes
+        check-modified-files-only: yes
+        base-branch: master

.github/workflows/tox.yml

@@ -67,30 +67,20 @@ jobs:
       fail-fast: false
       max-parallel: 4
       matrix:
-        tox-env: [flake8, pep517check, checkspelling, checklinks]
+        tox-env: [flake8, pep517check, checkspelling]
 
     env:
       TOXENV: ${{ matrix.tox-env }}
 
-    # Allow checklinks to fail
-    continue-on-error: ${{ matrix.tox-env == 'checklinks' }}
-
     steps:
     - uses: actions/checkout@v2
     - name: Setup Python
       uses: actions/setup-python@v2
       with:
         python-version: 3.7
-    - name: Setup Node
-      if: ${{ matrix.tox-env == 'checklinks' }}
-      uses: actions/setup-node@v2
-      with:
-        node-version: '10'
-        # cache: npm
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip tox
-        if [[ "$TOXENV" == 'checklinks' ]]; then npm install -g markdown-link-check; fi
         if [[ "$TOXENV" == 'checkspelling' ]]; then sudo apt-get install aspell aspell-en; fi
     - name: Run tox
       run: python -m tox

README.md

@@ -62,4 +62,3 @@ Code of Conduct
 
 Everyone interacting in the Python-Markdown project's codebases, issue trackers,
 and mailing lists is expected to follow the [Code of Conduct].
-