gh-149879: Fix test_venv on Cygwin

[vstinner]

venv now uses symlinks by default on Cygwin.

Fix test_zippath_from_non_installed_posix(): copy also the cygpython DLL.

• Issue: Enhance Cygwin support #149879

[vstinner]

@vsajip@FFY00: Are you ok with this change? It uses symlinks by default on Cygwin when using the Python API (venv module). The python -m venv command line interface already uses symlinks by default on Unix:

ifos.name=='nt':
use_symlinks=Falseelse:
use_symlinks=Truegroup=parser.add_mutually_exclusive_group()
group.add_argument('--symlinks', default=use_symlinks,
action='store_true', dest='symlinks',
help='Try to use symlinks rather than copies, ''when symlinks are not the default for ''the platform.')

[vstinner]

venv created with --copies doesn't work on Cygwin:

vstinner@win ~/cpython
$ ./python.exe -m venv ../env --without-pip --copies
vstinner@win ~/cpython
$ cd ../
vstinner@win ~
$ env/bin/python.exe -V
C:/cygwin64/home/vstinner/env/bin/python.exe: error while loading shared libraries: cygpython3.16d.dll: cannot open shared object file: No such file or directory

venv created with symlinks just works well:

vstinner@win ~/cpython
$ ./python.exe -m venv ../env --without-pip
vstinner@win ~/cpython
$ cd ..
vstinner@win ~
$ env/bin/python.exe -V
Python 3.16.0a0

[vstinner]

With this change, venv cannot be used in copy mode: running the venv python programs fails to locate the libpython DLL.

I wrote a different approach, copy the libpython DLL in the venv: PR gh-150483.