[mypyc] Use other arg instead of self for RHS type

[rheard]

Fixes #21568

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

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.

[p-sawicki]

thanks!