[PATCH 1 of 4] py3: ensure the proxied Windows fd doesn't escape by entering context manager

[Yuya Nishihara]

On Sat, 15 Dec 2018 15:04:24 -0500, Matt Harbison wrote:
> # HG changeset patch
> # User Matt Harbison <matt_harbison at yahoo.com>
> # Date 1544855178 18000
> #      Sat Dec 15 01:26:18 2018 -0500
> # Node ID 068910232e124a50a13fd7444844d9151db48d6b
> # Parent  5817c3b186a7799ecc3130493ba134b55cd4ba07
> py3: ensure the proxied Windows fd doesn't escape by entering context manager

Good catch. Queued these for stable because some of them will break things
if "with" statement is involved, and I can't be sure we have no such invocation.