A new class is added in a pyotp.contrib.steam submodule. It's a subclass of pyotp.TOTP and overrides the generate_otp method. (all as mentioned in the issue here)](https://github.com/pyauth/pyotp/issues/127#issuecomment-1019431149)))

Also added some tests and included the new class in the API Documentation section.

Co-authored-by: Andrey Kislyuk <kislyuk@gmail.com>