Must not use Pybind's implicit object-identity `__hash__`

**To Reproduce**
1. Create a file: islpy_hash.py as:

```python
import islpy as isl

a = isl.BasicSet("{[i,j]: 0<=i<=j<10}")

print(hash("hashing of islpy objects isn't Python compliant"))
print(hash(a))
print(hash(a))
```
2. Call it as `PYTHONHASHSEED=3 python islpy_hash.py`.

**Observed behavior**

```console
(py311_env) [line@line ~]$ PYTHONHASHSEED=3 python islpy_hash.py
-5820926828285319218
8778101896947
8778101896947
(py311_env) [line@line ~]$ PYTHONHASHSEED=3 python islpy_hash.py
-5820926828285319218
8727734557427
8727734557427
(py311_env) [line@line ~]$ PYTHONHASHSEED=3 python islpy_hash.py
-5820926828285319218
8787476382451
8787476382451
```

**Expected behavior**

The message printed to stdout must be the same across interpreter runs.