gh-149879: Fix test_c_stack_unwind on Cygwin

[vstinner]

On Cygwin, the Python library is called "cygpython3.16.dll".

• Issue: Enhance Cygwin support #149879

[vstinner]

Sadly, this change is not enough to fix all test_c_stack_unwind tests. A test still fails on Cygwin:

======================================================================
FAIL: test_manual_unwind_finds_expected_frames (test.test_c_stack_unwind.ManualStackUnwindTests.test_manual_unwind_finds_expected_frames) (env={'PYTHON_JIT': '0'})
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/cpython/Lib/test/test_c_stack_unwind.py", line 285, in test_manual_unwind_finds_expected_frames
    self.assertGreaterEqual(
    ~~~~~~~~~~~~~~~~~~~~~~~^
        python_frames,
        ^^^^^^^^^^^^^^
        STACK_DEPTH,
        ^^^^^^^^^^^^
        f"expected to find Python frames on {self.machine} with env {env}",
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
AssertionError: 4 not greater than or equal to 10 : expected to find Python frames on x86_64 with env {'PYTHON_JIT': '0'}