error[E0277]: `UnsafeCell<PyObject>` cannot be shared between threads safely
   --> tests/ui/not_send2.rs:8:12
    |
8   |         py.allow_threads(|| {
    |            ^^^^^^^^^^^^^ `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 because of the requirements on the impl of `Send` for `&&PyString`
    = note: required because it appears within the type `[closure@$DIR/tests/ui/not_send2.rs:8:26: 10:10]`
    = note: required because of the requirements on the impl of `Ungil` for `[closure@$DIR/tests/ui/not_send2.rs:8:26: 10:10]`
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`