fix(requirements): Fix requirements for async transport testing on python versions

GH-4601

Author: srothh

requirements-testing.txt

@@ -12,7 +12,8 @@ asttokens
 responses
 pysocks
 socksio
-httpcore[http2,asyncio]>=1.0 # From 1.0, httpcore async is optional
+httpcore[http2]; python_version < "3.8"
+httpcore[http2,asyncio]>=1.0; python_version >= "3.8" # asyncio support only supported for Python 3.8+
 setuptools
 freezegun
 Brotli