the following script:
```
import sys, types

def tr(frame, event, arg):
    print(frame, event, arg)
    return tr

sys.settrace(tr)
```
gives the output:
```
<frame object at 0x106c5cd48> call None
<frame object at 0x106c5cd48> exception (<class 'GeneratorExit'>, GeneratorExit(), <traceback object at 0x106cc1f88>)
<frame object at 0x106c5cd48> return None
```

This is due to Lib/types.py creating an async generator for the sole purpose of getting its type. I'll remove that reference after use to prevent the above message, which is probably benign but perhaps unnerving.

I am sure this change already was discussed in other issue, but I can not find that issue and I am not sure that there is a PR.

As it happens @cheryl.sabella has created a very useful data set of which files are impacted by open PRs: https://github.com/csabella/pulls

and as of 2019-02-18, the only open PR that relates to Lib/types.py is
https://github.com/python/cpython/pull/5068 on an unrelated matter

New changeset 0a6a412fb27d6874a0db5cc82a97f54d7c5fd0f2 by Serhiy Storchaka (Henry Chen) in branch 'master':
bpo-36091: Remove reference to async generator in Lib/types.py. (GH-11996)
https://github.com/python/cpython/commit/0a6a412fb27d6874a0db5cc82a97f54d7c5fd0f2

New changeset cd0416466f8a6d5333d6ea52f6907c39b8a6c3bc by Miss Islington (bot) in branch '3.7':
bpo-36091: Remove reference to async generator in Lib/types.py. (GH-11996)
https://github.com/python/cpython/commit/cd0416466f8a6d5333d6ea52f6907c39b8a6c3bc