Update pythoncapi_compat.h to fix building with PyPy3.11

Update `pythoncapi_compat.h` to upstream commit
c84545f0e1e21757d4901f75c47333d25a3fcff0, which includes fixes necessary
for Pillow to build against PyPy3.11.  Otherwise, it fails due to
duplicate declarations:

```
      In file included from src/encode.c:28:
      src/thirdparty/pythoncapi_compat.h:295:1: error: static declaration of ‘PyThreadState_GetInterpreter’ follows non-static declaration
        295 | PyThreadState_GetInterpreter(PyThreadState *tstate)
            | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
      In file included from /usr/include/pypy3.11/Python.h:80,
                       from src/encode.c:26:
      /usr/include/pypy3.11/pystate.h:35:33: note: previous declaration of ‘PyThreadState_GetInterpreter’ with type ‘PyInterpreterState *(PyThreadState *)’ {aka ‘struct _is *(struct _ts *)’}
         35 | PyAPI_FUNC(PyInterpreterState*) PyThreadState_GetInterpreter(PyThreadState *tstate);
            |                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Author: mgorny