Pin pytest-asyncio to 0.10.0 until event_loop priority issue is fixed

Awaited issue: pytest-dev/pytest-asyncio#154
Awaited fix: pytest-dev/pytest-asyncio#156

requirements.txt

@@ -4,10 +4,10 @@
 # Everything needed to develop (test, debug) the framework.
 coverage<5.0  # manual enforcement where pip fails, see #272.
 pytest-aiohttp
-pytest-asyncio
-pytest-mock>=1.11.1
+pytest-asyncio==0.10.0  # until fixed: https://github.com/pytest-dev/pytest-asyncio/issues/154
+pytest-mock
 pytest-cov
-pytest>=5.4.0
+pytest<5.4.0  # until fixed: https://github.com/pytest-dev/pytest-asyncio/issues/154
 aresponses
 asynctest
 freezegun