Cannot silence "unused 'type: ignore'" in version specific code

I need to execute some code only on specific Python version (it is the use of TextIOWrapper.reconfigure() added in Python 3.7, but in following example I will use more abstract code).

import sys

if sys.version_info >= (3, 7):
    x: int = 'a'

Since MyPy on Python 3.7 raises an error for it (actually because of the error in typeshed) I need to add # type: ignore:

import sys

if sys.version_info >= (3, 7):
    x: int = 'a'  # type: ignore

But now I will get an error unused 'type: ignore' comment when run MyPy with warn_unused_ignores = True on Python 3.6.

So I cannot use variant 1, because it is an error on 3.7, and cannot use variant 2, because it is an error on 3.6.

MyPy should not complain about unused 'type: ignore' comment in the code which it does not analyze.

MyPy version is 0.770.