When trying to impor _pyio from python3 in Cygwin, import fails with exception:
$ python3
Python 3.6.3 (default, Oct 31 2017, 19:00:36)
[GCC 6.4.0] on cygwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import _pyio
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.6/_pyio.py", line 17, in <module>
    from msvcrt import setmode as _setmode
ModuleNotFoundError: No module named 'msvcrt'

This issue is breaking cherrypy server (cheroot component):
https://github.com/cherrypy/cheroot/blob/86c6b246ec7cb704c6f96123556db1a083301634/cheroot/makefile.py#L5

Previous import is working in python 2.7:
$ python
Python 2.7.14 (default, Oct 31 2017, 21:12:13)
[GCC 6.4.0] on cygwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import _pyio
>>>

The msvcrt module is part of the standard library on Windows. Your cygwin installation may be incomplete.

By the way, why are you importing a private module? _pyio isn't a public API and shouldn't be used.

Presumably this issue does prevent us from testing _pyio on Cygwin, which is not ideal.  The outside usage of _pyio is definitely not supported, as Christian said, but this may be worth fixing anyway.

I am using not using _pyio module directly but I am using cherrypy server which is using this module (more precisely its component cheroot):
https://github.com/cherrypy/cheroot/blob/86c6b246ec7cb704c6f96123556db1a083301634/cheroot/makefile.py#L5

Regarding installation of cygwin environment, I can check it in other machine.

Could you please file a bug with cherrypy and get it fixed?

Cygwin not yet officially supported and stable. Erik is working hard on stable support for 3.7.

Right, so you should raise an issue with cherrypy :).  You're more likely to get a fix from them quickly, and their usage of _pyio is not correct anyway.

Also, note that Cygwin is not (anymore/yet) a fully-supported platform.  We're working towards that status, but aren't there yet.

This issue seems to duplicate #28459.

#28459 has a patch (implement _setmode() by ctypes on _pyio).  I'm not sure _setmode() will be used in other places in the future, but the patch just works at the moment.
In addition, I have an idea which implements C extension (imitation msvcrt or just cygwin).

Indeed, this is a duplicate of #28459.  I'll also update the existing patch to a pull request.