[mypyc] Use `other` arg instead of `self` for RHS type (#21569)

Fixes #21568 

In `gen_glue_ne_method`, mypyc currently chooses the generated RHS
argument type like this when strict dunder typing is enabled:

```python
rhs_type = eq_sig.args[0].type if strict_typing else object_rprimitive
```

However, the `__eq__` signature includes `self`, so `eq_sig.args[0]` is
the type of `self`, not the type of `other`.

---------

Co-authored-by: Piotr Sawicki <sawickipiotr@outlook.com>