rust-cpython: mark PySharedState as Sync so &'PySharedState can be Send

The goal is to store &'static PySharedState in $leaked struct, which allows
us to move the $leaked struct out of the macro. Currently, it depends on
inner.$data_member(py), which can't be generalized.

PySharedState is Sync because any mutation or read operation is synchronized
by the Python GIL, py: Python<'a>, which should guarantee that &'PySharedState
can be sent to another thread.

rust/hg-cpython/src/ref_sharing.rs

@@ -33,6 +33,10 b' pub struct PySharedState {'
     mutably_borrowed: Cell<bool>,
 }
 
+// &PySharedState can be Send because any access to inner cells is
+// synchronized by the GIL.
+unsafe impl Sync for PySharedState {}
+
 impl PySharedState {
     pub fn borrow_mut<'a, T>(
         &'a self,