Use python -m to run pytest in CI

Author: kalzoo

.github/workflows/ci.yml

@@ -14,8 +14,8 @@ jobs:
     runs-on: ${{ matrix.os }}-latest
     strategy:
       matrix:
-        os: [Ubuntu, MacOS, Windows]
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        os: [Ubuntu,]
+        python-version: [3.9]
         exclude:
           - os: Windows
             python-version: 3.6
@@ -66,4 +66,4 @@ jobs:
 
       - name: Execute test suite
         shell: bash
-        run: poetry run pytest tests/
+        run: poetry run python -m pytest tests/