gh-149879: Don't import msvcrt in _pyio on Cygwin

[vstinner]

The msvcrt extension module cannot be built on Cygwin.

• Issue: Enhance Cygwin support #149879

[vstinner]

This change fix a regression introduced by commit f0f55a0 in 2015. It does fix the issue reported in 2016: #69069 (comment).

[vstinner]

The macos-26 job failed with:

FAIL: test_trampoline_works_with_forks (test.test_perf_profiler.TestPerfTrampoline.test_trampoline_works_with_forks)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/runner/work/cpython/cpython/Lib/test/test_perf_profiler.py", line 145, in test_trampoline_works_with_forks
self.assertEqual(process.returncode, 0)
~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 11 != 0

That's the unrelated issue gh-149156.