gh-149879: Fix test_concurrent_futures on Cygwin

On Cygwin, skip tests using "forkserver" start method.

Author: vstinner

Lib/test/test_concurrent_futures/test_init.py

@@ -147,6 +147,8 @@ def test_spawn(self):
         self._test(ProcessPoolSpawnFailingInitializerTest)
 
     @support.skip_if_sanitizer("TSAN doesn't support threads after fork", thread=True)
+    @unittest.skipIf(sys.platform == "cygwin",
+                     "Forkserver is not available on Cygwin")
     def test_forkserver(self):
         self._test(ProcessPoolForkserverFailingInitializerTest)
 

Lib/test/test_concurrent_futures/util.py

@@ -135,7 +135,7 @@ def get_context(self):
             _check_system_limits()
         except NotImplementedError:
             self.skipTest("ProcessPoolExecutor unavailable on this system")
-        if sys.platform == "win32":
+        if sys.platform in ("win32", "cygwin"):
             self.skipTest("require unix system")
         if support.check_sanitizer(thread=True):
             self.skipTest("TSAN doesn't support threads after fork")