Link explicitly with libpython under Cygwin

As with native Windows Python, we need to explicitly link with
libpython under Cygwin too.

This fixes building extensions under Cygwin.

Author: lazka

newsfragments/5571.fixed.md

@@ -0,0 +1 @@
+Fix linking with libpython under Cygwin

pyo3-build-config/src/impl_.rs

@@ -890,6 +890,7 @@ pub fn is_linking_libpython_for_target(target: &Triple) -> bool {
         || target.operating_system == OperatingSystem::Aix
         || target.environment == Environment::Android
         || target.environment == Environment::Androideabi
+        || target.operating_system == OperatingSystem::Cygwin
         || !is_extension_module()
 }