bpo-35409: Ignore GeneratorExit in async_gen_athrow_throw (GH-14755)


Ignore `GeneratorExit` exceptions when throwing an exception into the `aclose` coroutine of an asynchronous generator.

https://bugs.python.org/issue35409
(cherry picked from commit 8e0de2a4808d7c2f4adedabff89ee64e0338790a)

Co-authored-by: Vincent Michel <vxgmichel@gmail.com>