Revert update of pytest-asyncio

Version 0.11 has an issue that causes us trouble, see
pytest-dev/pytest-asyncio#158

Author: Cito

setup.py

@@ -30,7 +30,9 @@
     extras_require={
         "test": [
             "pytest>=5.4,<5.5",
-            "pytest-asyncio>=0.11,<0.12",
+            # Note: tests do not work with pytest-asyncio 0.11, see
+            # https://github.com/pytest-dev/pytest-asyncio/issues/158
+            "pytest-asyncio>=0.10,<0.11",
             "pytest-cov>=2.8,<3",
             "jinja2>=2.11,<3",
             "yarl>1.4,<1.5",