feat(api): Make RESTManager generic on RESTObject class

Currently mixins like ListMixin are type hinted to return base
RESTObject instead of a specific class like `MergeRequest`.

The GetMixin and GetWithoutIdMixin solve this problem by defining
a new `get` method for every defined class. However, this creates
a lot of duplicated code.

Make RESTManager use `typing.Generic` as its base class and use
and assign the declared TypeVar to the `_obj_cls` attribute as
a type of the passed class.

Make both `_obj_cls` and `_path` attributes an abstract properties
so that type checkers can check that those attributes were properly
defined in subclasses. Mypy will only check then the class is
instantiated which makes non-final subclasses possible.
Unfortunately pylint will check the declarations not instantiations
so add `# pylint: disable=abstract-method` comments to all non-final
subclasses like ListMixin.

Make `_path` attribute always be `str` instead of sometimes `None`.
This eliminates unnecessary type checks.

Change all mixins like ListMixin or GetMixin to a subclass. This makes
the attribute declarations much cleaner as for example `_list_filters`
is now the only attribute defined by ListMixin.

Change SidekiqManager to not inherit from RESTManager and only
copy its `__init__` method. This is because SidekiqManager never
was a real manager and does not define `_path` or `_obj_cls`.

Delete `tests/unit/meta/test_ensure_type_hints.py` file as the `get`
method is no required to be defined for every class.

Signed-off-by: Igor Ponomarev <igor.ponomarev@collabora.com>