ui: stop using the `pycompat.open()` shim

The two instances here were already compatible with the py3 builtin function.

mercurial/ui.py

@@ -38,9 +38,6 from typing import (
 
 from .i18n import _
 from .node import hex
-from .pycompat import (
-    open,
-)
 
 from . import (
     color,