[SIP] Type declarations not added for operator overloads.

[Greger Stolt Nilsen]

Code like

`Vector operator+ (const Vector &other) const;`

does not get typing added to the .pyi-file, and it seems to be the same 
for all operator-overloads.

Expected type code would be something like

`def __add__(self, other: 'Vector') -> Vector ...`

If this is a hard problem to solve, is there a way to manually add the 
type signatures in the sip-files?

Best regards
Greger