gh-149879: Fix test_venv on Cygwin

[vstinner]

In copy mode, venv now also copies the cygpython DLL.

Fix test_zippath_from_non_installed_posix(): copy also the cygpython DLL.

• Issue: Enhance Cygwin support #149879

[vstinner]

See also my first attempt to fix test_venv: #150426 changes venv API to use symlinks by default.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Debian root 3.x (tier-1) has failed when building commit bc23530.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/345/builds/14354) and take a look at the build logs.
4. Check if the failure is related to this commit (bc23530) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/345/builds/14354

Failed tests:

• test_embed

Failed subtests:

• test_thread_state_ensure - test.test_embed.AuditingTests.test_thread_state_ensure

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/test_embed.py", line 2000, in test_thread_state_ensure
    self.run_embedded_interpreter("test_thread_state_ensure")
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/test_embed.py", line 138, in run_embedded_interpreter
    self.assertEqual(p.returncode, returncode,
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
                     "bad returncode %d, stderr is %r" %
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                     (p.returncode, err))
                     ^^^^^^^^^^^^^^^^^^^^
AssertionError: -6 != 0 : bad returncode -6, stderr is "_testembed: ./Programs/_testembed.c:2736: test_thread_state_ensure: Assertion `_Py_atomic_load_int(&data.done) == 1' failed.\n"


Traceback (most recent call last):
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/support/__init__.py", line 938, in gc_collect
    gc.collect()
    ~~~~~~~~~~^^
ResourceWarning: unclosed file <_io.FileIO name=11 mode='wb' closefd=True>