Pin pytest-asyncio

See pytest-dev/pytest-asyncio#154.

setup.py

@@ -31,7 +31,7 @@
          'test': '''\
             coverage[toml] >=5
             pytest         >=5
-            pytest-asyncio
+            pytest-asyncio ==0.10.0
             aresponses     ~=2.0
          '''},
       python_requires='~=3.7',