Fix locals collisions with shell variables

Fixes issues with namespace collisions, e.g. :
```python
cfg = 'trap'
%config IPCompleter.use_jedi = False
```
Causes: `AttributeError: 'str' object has no attribute 'IPCompleter'`

fixes https://github.com/ipython/ipython/issues/11835

IPython/core/magics/config.py

@@ -1,158 +1,158 b''
 """Implementation of configuration-related magic functions.
 """
 #-----------------------------------------------------------------------------
 #  Copyright (c) 2012 The IPython Development Team.
 #
 #  Distributed under the terms of the Modified BSD License.
 #
 #  The full license is in the file COPYING.txt, distributed with this software.
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 
 # Stdlib
 import re
 
 # Our own packages
 from IPython.core.error import UsageError
 from IPython.core.magic import Magics, magics_class, line_magic
 from logging import error
 
 #-----------------------------------------------------------------------------
 # Magic implementation classes
 #-----------------------------------------------------------------------------
 
 reg = re.compile(r'^\w+\.\w+$')
 @magics_class
 class ConfigMagics(Magics):
 
     def __init__(self, shell):
         super(ConfigMagics, self).__init__(shell)
         self.configurables = []
 
     @line_magic
     def config(self, s):
         """configure IPython
 
             %config Class[.trait=value]
 
         This magic exposes most of the IPython config system. Any
         Configurable class should be able to be configured with the simple
         line::
 
             %config Class.trait=value
 
         Where `value` will be resolved in the user's namespace, if it is an
         expression or variable name.
 
         Examples
         --------
 
         To see what classes are available for config, pass no arguments::
 
             In [1]: %config
             Available objects for config:
                 TerminalInteractiveShell
                 HistoryManager
                 PrefilterManager
                 AliasManager
                 IPCompleter
                 DisplayFormatter
 
         To view what is configurable on a given class, just pass the class
         name::
 
             In [2]: %config IPCompleter
             IPCompleter options
             -----------------
             IPCompleter.omit__names=<Enum>
                 Current: 2
                 Choices: (0, 1, 2)
                 Instruct the completer to omit private method names
                 Specifically, when completing on ``object.<tab>``.
                 When 2 [default]: all names that start with '_' will be excluded.
                 When 1: all 'magic' names (``__foo__``) will be excluded.
                 When 0: nothing will be excluded.
             IPCompleter.merge_completions=<CBool>
                 Current: True
                 Whether to merge completion results into a single list
                 If False, only the completion results from the first non-empty
                 completer will be returned.
             IPCompleter.limit_to__all__=<CBool>
                 Current: False
                 Instruct the completer to use __all__ for the completion
                 Specifically, when completing on ``object.<tab>``.
                 When True: only those names in obj.__all__ will be included.
                 When False [default]: the __all__ attribute is ignored
             IPCompleter.greedy=<CBool>
                 Current: False
                 Activate greedy completion
                 This will enable completion on elements of lists, results of
                 function calls, etc., but can be unsafe because the code is
                 actually evaluated on TAB.
 
         but the real use is in setting values::
 
             In [3]: %config IPCompleter.greedy = True
 
         and these values are read from the user_ns if they are variables::
 
             In [4]: feeling_greedy=False
 
             In [5]: %config IPCompleter.greedy = feeling_greedy
 
         """
         from traitlets.config.loader import Config
         # some IPython objects are Configurable, but do not yet have
         # any configurable traits.  Exclude them from the effects of
         # this magic, as their presence is just noise:
         configurables = sorted(set([ c for c in self.shell.configurables
                                      if c.__class__.class_traits(config=True)
                                      ]), key=lambda x: x.__class__.__name__)
         classnames = [ c.__class__.__name__ for c in configurables ]
 
         line = s.strip()
         if not line:
             # print available configurable names
             print("Available objects for config:")
             for name in classnames:
                 print("    ", name)
             return
         elif line in classnames:
             # `%config TerminalInteractiveShell` will print trait info for
             # TerminalInteractiveShell
             c = configurables[classnames.index(line)]
             cls = c.__class__
             help = cls.class_get_help(c)
             # strip leading '--' from cl-args:
             help = re.sub(re.compile(r'^--', re.MULTILINE), '', help)
             print(help)
             return
         elif reg.match(line):
             cls, attr = line.split('.')
             return getattr(configurables[classnames.index(cls)],attr)
         elif '=' not in line:
             msg = "Invalid config statement: %r, "\
                   "should be `Class.trait = value`."
             
             ll = line.lower()
             for classname in classnames:
                 if ll == classname.lower():
                     msg = msg + '\nDid you mean %s (note the case)?' % classname
                     break
 
             raise UsageError( msg % line)
 
         # otherwise, assume we are setting configurables.
         # leave quotes on args when splitting, because we want
         # unquoted args to eval in user_ns
         cfg = Config()
-        exec("cfg."+line, locals(), self.shell.user_ns)
+        exec("cfg."+line, self.shell.user_ns, locals())
 
         for configurable in configurables:
             try:
                 configurable.update_config(cfg)
             except Exception as e:
                 error(e)