fix(dependencies): Remove version constraint from httpcore

GH-4601

Author: srothh

requirements-testing.txt

@@ -13,7 +13,7 @@ responses
 pysocks
 socksio
 httpcore[http2]; python_version < "3.8"
-httpcore[http2,asyncio]>=1.0; python_version >= "3.8" # asyncio support only supported for Python 3.8+
+httpcore[http2,asyncio]; python_version >= "3.8" # asyncio support only supported for Python 3.8+
 setuptools
 freezegun
 Brotli