Issue 34536: Enum._missing_ doesn't raise TypeError when a non-Enum object is a returned

In a conversation with Ethan Furman (the author of the enum module) I've been informed that Enum automatically checks whether the _missing_ method has returned an instance of the correct class, and raises a TypeError if not. (Link: https://stackoverflow.com/a/52064774/1222951)

However, this simply does not happen.

```
import enum

class MyEnum(enum.Enum):
    FOO = "foo"

    @classmethod
    def _missing_(cls, value):
        return 5

print(MyEnum('bar'))  # output: 5
```

New changeset 019f0a0cb85ebc234356415f3638b9bd77528e55 by Ethan Furman in branch 'master':
bpo-34536: raise error for invalid _missing_ results (GH-9147)
https://github.com/python/cpython/commit/019f0a0cb85ebc234356415f3638b9bd77528e55

I wrote PR 9978 to backport the fix to Python 3.7.

Should the fix be backported to Python 3.6 as well?

Change by STINNER Victor:

> pull_requests: +9319

Why does the above say 9319 when the PR is 9978?

New changeset 0f2fc8bee0b435ee2934751264196db30d16ed8a by Ethan Furman (Victor Stinner) in branch '3.7':
bpo-34536: raise error for invalid _missing_ results (GH-9147) (GH-9978)
https://github.com/python/cpython/commit/0f2fc8bee0b435ee2934751264196db30d16ed8a

> Why does the above say 9319 when the PR is 9978?

Roundup (the software running bugs.python.org) logs its own internal identifier for pull requests. Following https://bugs.python.org/pull_request9320 may help you to understand :-)

New changeset 4acf6c9d4be77b968fa498569d7a1545e5e77344 by Ethan Furman (Victor Stinner) in branch 'master':
bpo-34536: Cleanup test_enum imports (GH-9979)
https://github.com/python/cpython/commit/4acf6c9d4be77b968fa498569d7a1545e5e77344

messages: + msg328090

nosy: + ethan.furman