move testing requirements to requirements_test.txt, add pytest-asyncio for azure pipelines (#20)

azure-pipelines.yml

@@ -23,7 +23,7 @@ steps:
 - script: |
     python -m pip install --upgrade pip
     pip install -r requirements.txt
-    pip install pytest pytest-azurepipelines pytest-cov
+    pip install -r requirements_test.txt
   displayName: 'Install dependencies'
 
 - script: |