Special case method names in the ContextMap namespaces

[jsbueno]

There are no provisions right now to separate the MutableMapping method names (copy, keys, etc...) from context-variable keys - (the current behavior is even undefined, as in "I don't know what would happen if one is to use these keys" .

The suggestion is to separate a reserved attribute list as a class attribute (with the _et_ prefix, which should be reserved anyway) and give some special treatment to those. They colliding names should then just work on ContextMap instances, and maybe error with AttributeError on ContextLocal distances (there are no public methods there yet, but maybe copy_context and run should be reserved for future implementation of the functionality available in contextvars.Context)