Mingw style Python uses '\r\n' by default for newlines. This is
annoying, because it means that when a GHC developer on Windows uses
mingw Python to `make accept` a test, every single line of the
.stderr file is touched. This makes it difficult to spot the real
changes, and it leads to unnecessary git history bloat.

Prevent this from happening by using io.open instead of open.
See `Note [Universal newlines]`

Reviewed by: Phyx

Differential Revision: https://phabricator.haskell.org/D2342