Try to avoid refcount clone when importing PyObjectStore

[kylebarron]

I don't think this is possible:

error: lifetime may not live long enough
  --> pyo3-object_store/src/store.rs:20:28
   |
18 | impl<'py> FromPyObject<'py> for PyObjectStore<'py> {
   |      --- lifetime `'py` defined here
19 |     fn extract_bound(ob: &Bound<'py, PyAny>) -> PyResult<Self> {
   |                          - let's call the lifetime of this reference `'1`
20 |         if let Ok(store) = ob.downcast::<PyS3Store>() {
   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'1` must outlive `'py`