Suppress exception chaining in colormap lookup.

Replace
```
Traceback (most recent call last):
  File "../matplotlib/cm.py", line 131, in __getitem__
    return self._cmaps[item].copy()
KeyError: 'foo'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<string>", line 4, in <module>
  File ".../matplotlib/cm.py", line 133, in __getitem__
    raise KeyError(f"{item!r} is not a known colormap name")
KeyError: "'foo' is not a known colormap name"
```
by just the latter block.

Author: anntzer

lib/matplotlib/cm.py

@@ -130,7 +130,7 @@ def __getitem__(self, item):
         try:
             return self._cmaps[item].copy()
         except KeyError:
-            raise KeyError(f"{item!r} is not a known colormap name")
+            raise KeyError(f"{item!r} is not a known colormap name") from None
 
     def __iter__(self):
         return iter(self._cmaps)