[issue34850] Emit a syntax warning for "is" with a literal

[Anthony Sottile]

Anthony Sottile <asottile at umich.edu> added the comment:

Should this also produce warnings for `list` / `dict` / `set` literals?

```
$ python3.8
Python 3.8.0a3 (default, Mar 27 2019, 03:46:44) 
[GCC 7.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> x is []
False
>>> x is {}
False
>>> x is {1, 2}
False
```

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nosy: +Anthony Sottile

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<https://bugs.python.org/issue34850>
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