package/python3: do not install MS Windows installers on the target

package/python3: do not install MS Windows installers on the target

Buildroot generates Linux systems, so there is no need to have
MS Windows executables in there.

This reduces the target filesystem size by about 2MB:
$ du -hcs usr/lib/python3.8/distutils/command/wininst-*.exe
60K usr/lib/python3.8/distutils/command/wininst-6.0.exe
64K usr/lib/python3.8/distutils/command/wininst-7.1.exe
60K usr/lib/python3.8/distutils/command/wininst-8.0.exe
192K    usr/lib/python3.8/distutils/command/wininst-9.0.exe
220K    usr/lib/python3.8/distutils/command/wininst-9.0-amd64.exe
188K    usr/lib/python3.8/distutils/command/wininst-10.0.exe
220K    usr/lib/python3.8/distutils/command/wininst-10.0-amd64.exe
448K    usr/lib/python3.8/distutils/command/wininst-14.0.exe
576K    usr/lib/python3.8/distutils/command/wininst-14.0-amd64.exe
2,0M    total

Signed-off-by: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>