Update ci.yml

Removed specific reference to Python 3.7

Author: harry093

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v2
       with:
-        python-version: '3.8'
+        python-version: '3.x'
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip