Drop async_generator requirement (#2228)

Co-authored-by: Marcelo Trylesinski <[email protected]>

Author: lovelydinosaur

setup.py

@@ -61,7 +61,6 @@ def get_packages(package):
         "sniffio",
         "rfc3986[idna2008]>=1.3,<2",
         "httpcore>=0.14.5,<0.15.0",
-        "async_generator; python_version < '3.7'"
     ],
     extras_require={
         "http2": "h2>=3,<5",