Upstream fix for bpo-39386 breaks Stackless test case test_pickle.TestAsyncGenPickling

[akruis]

Commit a96e06d (branch main) / b76d5e9 (branch 3.7) breaks test_pickle.TestAsyncGenPickling. The test fails with
RuntimeError: cannot reuse already awaited __anext__()/asend().
The exception is caused by reusing an already awaited anext().

The fix is trivial.

[akruis]

Fixed in branch 3.7-slp and 3.8-slp. In main-slp the fix can only be applied after merging commit a96e06d from upstream. Then cherry-pick -x commit c3f61b0