error[E0277]: `UnsafeCell<PyObject>` cannot be shared between threads safely
  --> tests/ui/not_send2.rs:8:26
   |
8  |           py.allow_threads(|| {
   |  ____________-------------_^
   | |            |
   | |            required by a bound introduced by this call
9  | |             println!("{:?}", string);
10 | |         });
   | |_________^ `UnsafeCell<PyObject>` cannot be shared between threads safely
   |
   = help: within `&PyString`, the trait `Sync` is not implemented for `UnsafeCell<PyObject>`
   = note: required because it appears within the type `PyAny`
   = note: required because it appears within the type `PyString`
   = note: required because it appears within the type `&PyString`
   = note: required for `&&PyString` to implement `Send`
note: required because it's used within this closure
  --> tests/ui/not_send2.rs:8:26
   |
8  |         py.allow_threads(|| {
   |                          ^^
   = note: required for `[closure@$DIR/tests/ui/not_send2.rs:8:26: 8:28]` to implement `Ungil`
note: required by a bound in `pyo3::Python::<'py>::allow_threads`
  --> src/marker.rs
   |
   |         F: Ungil + FnOnce() -> T,
   |            ^^^^^ required by this bound in `pyo3::Python::<'py>::allow_threads`