Enable the pylint check "no-else-return" and fix the errors detected.

Enable the pylint check: "attribute-defined-outside-init" and fix
errors detected.

Enable the pylint check "raise-missing-from" and fix errors detected.

Move the `validate_attrs` function to be inside the `RequiredOptional`
class. It makes sense for it to be part of the class as it is working
on data related to the class.

add exclusive tuple to RequiredOptional data class to support for
mutually exclusive attributes

consolidate _check_missing_create_attrs and _check_missing_update_attrs
from mixins.py into _validate_attrs in utils.py

change _create_attrs in board list manager classes from
required=('label_ld',) to
exclusive=('label_id','asignee_id','milestone_id')

closes https://github.com/python-gitlab/python-gitlab/issues/1897

By having `RequiredOptional` in the `gitlab.base` module it makes it
difficult with circular imports. Move it to the `gitlab.types`
module which has no dependencies on any other gitlab module.

Commit f0152dc3cc9a42aa4dc3c0014b4c29381e9b39d6 removed situation
where `get()` in a `GetWithoutIdMixin` based class could return `None`

Update the type-hints to no longer return `Optional` AKA `None`

`as_list=False` is confusing as it doesn't explain what is being
returned. Replace it with `iterator=True` which more clearly explains
to the user that an iterator/generator will be returned.

This maintains backward compatibility with `as_list` but does issue a
DeprecationWarning if `as_list` is set.