Enable GILProtected access via PyVisit

Closes PyO3#3615

This simply adds a new method which uses the existence of a `PyVisit`
object as proof that the GIL is held instead of a `Python` object. This
allows `GILProtected` to be used in instances where contained Python
objects need to participate in garbage collection. Usage in this
situation should be valid since no Python calls are made and this does
not provide any additional mechanism for accessing a `Python` object.

src/sync.rs

@@ -1,5 +1,5 @@
 //! Synchronization mechanisms based on the Python GIL.
-use crate::{types::PyString, types::PyType, Py, PyErr, Python};
+use crate::{types::PyString, types::PyType, Py, PyErr, Python, PyVisit};
 use std::cell::UnsafeCell;
 
 /// Value with concurrent access protected by the GIL.
@@ -37,6 +37,11 @@ impl<T> GILProtected<T> {
     pub fn get<'py>(&'py self, _py: Python<'py>) -> &'py T {
         &self.value
     }
+
+    /// Gain access to the inner value by giving proof that garbage collection is happening.
+    pub fn get_visit<'py>(&'py self, _visit: PyVisit<'py>) -> &'py T {
+        &self.value
+    }
 }
 
 unsafe impl<T> Sync for GILProtected<T> where T: Send {}