Option to disable import replacement

[agronholm]

The typing_extensions library contains more up to date versions of some functions and should not be replaced with the corresponding stdlib import. But as the tool cannot know when I actually need these newer versions, there should preferably be an option to prevent the replacement of imports, much like with # noreorder.

A practical example, one I ran into, is the get_type_hints() function which provides the include_extras parameter not found in the stdlib version.

[asottile]

in such cases we usually update the target version instead -- for example #289

feel free to send a patch