Currently sys.stdin et al. are typed as TextIO, not TextIOWrapper. This would need to be changed as well, although I am not sure that this does not cause complications. But adding TextIOWrapper.reconfigure() would be welcome in any case.

For the following code

import sys, io

if isinstance(sys.stdout, io.TextIOWrapper) and sys.version_info >= (3, 7):
    sys.stdout.reconfigure(errors="replace")

I get the error:

reconfigure37.py:4: error: "TextIOWrapper" has no attribute "reconfigure"

MyPy understand that sys.stdout is a TextIOWrapper instance.

What is worse -- I cannot silence the error by adding # type: ignore, because on Python 3.6 I get error: unused 'type: ignore' comment.

I haven't touched MyPy in many months but your platform check might be too complex for https://mypy.readthedocs.io/en/stable/common_issues.html#python-version-and-system-platform-checks to understand. Try splitting it into two ifs?

FWIW, I just checked the following with mypy 0.770 using --warn-unused-ignores under both Python 3.6.9 and 3.7.4 and it works or me:

import sys, io

if isinstance(sys.stdout, io.TextIOWrapper) and sys.version_info >= (3, 7):
    sys.stdout.reconfigure(errors="replace")  # type: ignore

That said, we'll gladly accept a PR that adds TextIOWrapper.reconfigure() :)

It is the same if split on two ifs.