#893 contains a tricky bug. The following is incorrect:

```python
def stream_register(stream: int, flags: int) -> None:
    cdef CUstream cpp_stream = <CUstream>stream
```
`stream` is a Python object. The cast applies to the address of this Python object, instead of the underlying integer value. This surprisingly does not trigger a compile error.

There are two ways to fix this bug:
- Method 1: double casting. The additional cast to `uintptr_t` extracts the underlying integer value of `stream` correctly.
```python
def stream_register(stream: int, flags: int) -> None:
    cdef CUstream cpp_stream = <CUstream><uintptr_t>stream
```

- Method 2: `uintptr_t` as type hint (used in this PR). `uintptr_t` is not just a type hint; it actually causes `stream` to be a C object.
```python
def stream_register(stream: uintptr_t, flags: int) -> None:
    cdef CUstream cpp_stream = <CUstream>stream
```
For both method, `uintptr_t` must be `cimport`-ed into the source file.

Authors:
  - Tianyu Liu (https://github.com/kingcrimsontianyu)

Approvers:
  - Lawrence Mitchell (https://github.com/wence-)

URL: #904