Make teste work with Python 3.6 again

Note that pytest-asyncio 0.17 is not supported for Python 3.6.

Author: Cito

setup.py

@@ -5,7 +5,7 @@
 
 tests_requires = [
     "pytest>=6.2,<6.3",
-    "pytest-asyncio>=0.17,<1",
+    "pytest-asyncio>=0.16,<1",
     "pytest-cov>=3,<4",
     "aiohttp>=3.8,<4",
     "Jinja2>=2.11,<3",