Rollup merge of rust-lang#45120 - johnthagen:none-identity-test, r=sfackler

Use identity operator `is` when comparing to None

This is very minor, but idiomatic Python code uses `is` for comparisons to `None`. This is because semantically we want to compare to the "identity" of `None`, not its value.

See [PEP8 for details](https://www.python.org/dev/peps/pep-0008/#programming-recommendations).

Author: kennytm

src/bootstrap/bootstrap.py

@@ -498,7 +498,7 @@ def get_toml(self, key):
 
         If the key does not exists, the result is None:
 
-        >>> rb.get_toml("key3") == None
+        >>> rb.get_toml("key3") is None
         True
         """
         for line in self.config_toml.splitlines():