[issue34608] gc.get_referrers behavior change 3.6 to 3.7

[Bryan]

New submission from Bryan <bryanv at ftml.net>:

When called on a local object inside a function, gc.get_referrers no longer returns a Frame as one of the references. I could not find anything in the release notes or changeling that indicated that this is an intentional change. 

The following script generates different output when run on Python 3.6 vs Python 3.7 (on linux, OSX, or Windows):

```
# referrers.py
import gc, sys

class FakeMod(object): pass

extra = []

def test():
    mod = FakeMod()
    extra.append(mod)
    referrers = gc.get_referrers(mod)
    print(".".join(str(x) for x in sys.version_info[:3]), ":", len(referrers), referrers)

test()
```

Output:

~ master*
(py37) ❯ python test.py
3.7.0 : 1 [[<__main__.FakeMod object at 0x10b65e320>]]

~ master*
(base) ❯ python test.py
3.6.6 : 2 [[<__main__.FakeMod object at 0x106f3ea90>], <frame object at 0x10684b648>]

----------
components: Library (Lib)
messages: 324771
nosy: bryevdv
priority: normal
severity: normal
status: open
title: gc.get_referrers behavior change 3.6 to 3.7
versions: Python 3.7

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34608>
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