update mkdocs to be compatible with python 3.7+

fix #261

.github/workflows/deploy.yml

@@ -14,9 +14,7 @@ jobs:
     - name: install dependencies
       shell: bash
       run: |
-        pip install PyYAML==5.1.2
-        pip install mkdocs==0.16.3
-        python setup.py install
+        pip install mkdocs==1.4.2 python-markdown-math==0.8
     - name: build site
       shell: bash
       run: mkdocs build --clean