Update docs for "Import cycles" to refer to TYPE_CHECKING #2178

On the following page:

• http://mypy.readthedocs.io/en/latest/common_issues.html#import-cycles

It is mentioned:

It is possible that in the future, mypy will change its dead code analysis and this trick will stop working. We will then offer an alternative, e.g. a constant defined by the typing module that is False at runtime but True while type checking.

The aforementioned constant now exists in the form of typing.TYPE_CHECKING. It might be useful to update the docs to refer to it, including in the example.

I could put a docs patch together if this is the correct approach.