fix: pin 'aoihttp < 3.7.0dev' (#634)

Working around breaking change in 3.7.0.  See:

https://github.com/pnuckowski/aioresponses/issues/173
diff --git a/system_tests/noxfile.py b/system_tests/noxfile.py
index a039228..0f852ea 100644
--- a/system_tests/noxfile.py
+++ b/system_tests/noxfile.py
@@ -168,7 +168,7 @@
 
 # Test sesssions
 
-TEST_DEPENDENCIES_ASYNC = ["aiohttp", "pytest-asyncio", "nest-asyncio"]
+TEST_DEPENDENCIES_ASYNC = ["aiohttp < 3.7.0dev", "pytest-asyncio", "nest-asyncio"]
 TEST_DEPENDENCIES_SYNC = ["pytest", "requests"]
 PYTHON_VERSIONS_ASYNC = ["3.7"]
 PYTHON_VERSIONS_SYNC = ["2.7", "3.7"]