Only running pytest on 3.7 (race conditions)

Multiple tests in parallel v(3.6, 3.7, 3.8) cause race conditions due to them hitting the same API for the same user, leading to AssertionErrors.

Since GitHub doesn't offer a way to run sequential workflows, testing is limited to 3.7

.github/workflows/pytest.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.7]
 
     steps:
     - uses: actions/checkout@v2