Specify Python versions as a workaround for the lxml's issue on Python 3.9

See https://bugs.launchpad.net/lxml/+bug/1899830

Author: kmyk

.github/workflows/format.yml

@@ -11,6 +11,8 @@ jobs:
 
     - name: Set up Python
       uses: actions/setup-python@v2
+      with:
+        python-version: 3.8
 
     - name: Install dependencies
       run: pip3 install .[dev]

.github/workflows/pypi.yml

@@ -13,6 +13,8 @@ jobs:
 
     - name: Set up Python
       uses: actions/setup-python@v2
+      with:
+        python-version: 3.8
 
     - name: Install dependencies
       run: |