Drop support for Python 2.7, 3.5, 3.6

Python 2.7 is EOL
Python 3.5 is EOL
Python 3.6 is EOL in 5 days

.github/workflows/test.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        py_version: [2.7, 3.5, 3.6, 3.7]
+        py_version: [3.7, 3.8, 3.9, 3.10]
 
     steps:
     - uses: actions/checkout@v1
@@ -17,7 +17,5 @@ jobs:
       uses: actions/setup-python@v1
       with:
         python-version: ${{ matrix.py_version }}
-    - name: Install mock for Python 2.7
-      run: pip install mock
     - name: Run tests
       run: python setup.py test