CI tests: Set PYTHONIOENCODING=utf-8 on Windows

This is to work-around a tox encoding bug.

Ref: tox-dev/tox#1550

Author: dairiki

.github/workflows/ci.yml

@@ -111,6 +111,13 @@ jobs:
         if: startsWith(runner.os, 'windows') && matrix.python == '3.9'
         run: choco install --no-progress --timeout 600 imagemagick.tool ffmpeg
         continue-on-error: true
+      - name: Workaround for UnicodeDecodeError from tox on Windows
+        # Refs:
+        #   https://github.com/lektor/lektor/pull/933#issuecomment-923107580
+        #   https://github.com/tox-dev/tox/issues/1550
+        if: startsWith(runner.os, 'windows')
+        shell: bash
+        run: echo "PYTHONIOENCODING=utf-8" >> $GITHUB_ENV
       - name: Install python dependencies
         run: python -m pip install codecov tox
       - name: Run python tests