Thank you for the fast response!

Both versions are distributed via msys I did not build them myself and installed 3.7 via pacman (today).

$ python3.7
Python 3.7.0 (default, Jul 12 2018, 11:44:33)  [GCC 7.3.0 64 bit (AMD64)] on win32

I only found out about this issue after someone else complained that GHC's testsuite was broken with 3.7. After upgrading mine it broke for me as well and we narrowed it down to this issue.

I've tested the version from python.org in a windows shell as well which works so I can confirm your results there.
Python 3.7.0 (v3.7.0:1bf9cc5093, Jun 27 2018, 04:59:51) [MSC v.1914 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.

I've also tried running the version from python.org but in the msys shell which also works.

So it seems to be an issue with the version distributed by msys!
I'm not familiar with the details of how msys composes their packages. (If they distribute an official build, build their own or what not).

But this seems related: https://github.com/Alexpux/MINGW-packages/issues/4062
I will link this thread there and hope this leads somewhere.