s/this class/all classes provided by this package

A return type of `int` is also implicitly assumed in `HOTP` (see [here](https://github.com/nathforge/pyotp/blob/master/src/pyotp/hotp.py#L10))

If you can show me one instance of a `generate_otp` override with a non-numeric return type in the wild, I'll eat my shoes.