Use custom object for infinity point instead of `None`

### What is wrong?
For #89, discussion offline:

If I call mypy --strict --follow-imports=silent --ignore-missing-imports py_ecc (without `--no-strict-optional`), then 50+ errors about "None" infinity point would show up

If I call the same command with your PR, the errors:

```
py_ecc/bn128/bn128_curve.py:63: error: 'None' object is not iterable
py_ecc/bn128/bn128_curve.py:75: error: 'None' object is not iterable
py_ecc/bls12_381/bls12_381_curve.py:66: error: 'None' object is not iterable
py_ecc/bls12_381/bls12_381_curve.py:78: error: 'None' object is not iterable
```

But it's already handled with `if is_inf(pt): return pt`

None is really tricky in typing, perhaps there should be a special constant object for infinity point?

### How can it be fixed