PyMemoryView_FromMemory is part of stable ABI but the flag constants (PyBUF_READ, etc.) are not

# Feature or enhancement

I'd rather not write:

```c
#ifndef PyBUF_READ
#define PyBUF_READ 0x100
#endif
```

and instead be able to rely on these even in stable abi mode


relevant error message:

```
whatever.c:1:30: error: ‘PyBUF_READ’ undeclared (first use in this function)
```


### Linked PRs
* gh-100018
* gh-100020