error[E0277]: `*mut pyo3::Python<'static>` 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 | |         });
   | |_________^ `*mut pyo3::Python<'static>` cannot be shared between threads safely
   |
   = help: within `pyo3::Bound<'_, PyString>`, the trait `Sync` is not implemented for `*mut pyo3::Python<'static>`, which is required by `{closure@$DIR/tests/ui/not_send2.rs:8:26: 8:28}: Ungil`
note: required because it appears within the type `PhantomData<*mut pyo3::Python<'static>>`
  --> $RUST/core/src/marker.rs
   |
   | pub struct PhantomData<T: ?Sized>;
   |            ^^^^^^^^^^^
note: required because it appears within the type `impl_::not_send::NotSend`
  --> src/impl_/not_send.rs
   |
   | pub(crate) struct NotSend(PhantomData<*mut Python<'static>>);
   |                   ^^^^^^^
   = note: required because it appears within the type `(&pyo3::gil::GILGuard, impl_::not_send::NotSend)`
note: required because it appears within the type `PhantomData<(&pyo3::gil::GILGuard, impl_::not_send::NotSend)>`
  --> $RUST/core/src/marker.rs
   |
   | pub struct PhantomData<T: ?Sized>;
   |            ^^^^^^^^^^^
note: required because it appears within the type `pyo3::Python<'_>`
  --> src/marker.rs
   |
   | pub struct Python<'py>(PhantomData<(&'py GILGuard, NotSend)>);
   |            ^^^^^^
note: required because it appears within the type `pyo3::Bound<'_, PyString>`
  --> src/instance.rs
   |
   | pub struct Bound<'py, T>(Python<'py>, ManuallyDrop<Py<T>>);
   |            ^^^^^
   = note: required for `&pyo3::Bound<'_, PyString>` to implement `Send`
note: required because it's used within this closure
  --> tests/ui/not_send2.rs:8:26
   |
8  |         py.allow_threads(|| {
   |                          ^^
   = note: required for `{closure@$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
   |
   |     pub fn allow_threads<T, F>(self, f: F) -> T
   |            ------------- required by a bound in this associated function
   |     where
   |         F: Ungil + FnOnce() -> T,
   |            ^^^^^ required by this bound in `Python::<'py>::allow_threads`