Remove unnecessary object class derive in docs

[johnthagen]

Since Python 2.7 has been dropped, the docs can be simplified and the unnecessary object base class removed as this is default behavior in Python 3.

[jairhenrique]

@johnthagen what do you think about removing on these files too?

rest_framework/schemas/generators.py
146:class BaseSchemaGenerator(object):

docs/api-guide/validators.md
285:    class MultipleOf(object):

docs/api-guide/fields.md
606:    class Color(object):

docs/api-guide/generic-views.md
322:    class MultipleFieldLookupMixin(object):

[johnthagen]

@jairhenrique Done. If whoever merges could squash at that time through the GitHub UI, that'd be great.

[auvipy]

yeah, good change! if you can change the other files as well that would be really great.

[johnthagen]

@auvipy I changed everything suggested by @jairhenrique. Were there other files you wanted changed?

[rpkilby]

Thanks @johnthagen!

If whoever merges could squash at that time through the GitHub UI, that'd be great.

We pretty much always use squash merge, so you're good 👍