Update package info for system python3 on Cygwin

[slel]

Sage 9.5.beta9 fails to use system Python 3.9 on Cygwin,
unless configured with

./configure --with-python=/usr/bin/python3

Upstream report:
https://cygwin.com/pipermail/cygwin/2021-December/250302.html

Upstream: Fixed upstream, in a later stable release.

CC: @mkoeppe @slel @darijgr

Component: porting: Cygwin

Author: Matthias Koeppe

Branch/Commit: 65418de

Reviewer: Darij Grinberg

Issue created by migration from https://trac.sagemath.org/ticket/33078

[mkoeppe]

comment:1

logs please

[mkoeppe]

Branch: u/mkoeppe/use_system_python_3_on_cygwin

[mkoeppe]

Author: Matthias Koeppe

[mkoeppe]

New commits:

9cba975

build/pkgs/{_prereq,python3}/distros/cygwin.txt: Switch to python 3.9

[mkoeppe]

Commit: 9cba975

[slel]

comment:4

Fails here. Something may be wrong with my Cygwin.

Before getting Python 3.9 there, Sage would build
using system Python 3.8.12.

It now deems system Python 3.9 unusable, and builds
a lacking Python, see #33079.

Logs for failed build of Sage 9.5.beta9 + #33078:

• https://aij.ovh/sage/logs/slel-2021-12-27-cyg-sage-9-5-b9-33078.zip

[slel]

Description changed:

--- 
+++ 
@@ -2,6 +2,6 @@
 unless configured with
 
 ```
-./configure --with-python3=/usr/bin/python3
+./configure --with-python=/usr/bin/python3
 ```