chore: rename `gitlab/__version__.py` to `gitlab/_version.py`

It is confusing to have a `gitlab/__version__.py` because we also
create a variable `gitlab.__version__` which can conflict with
`gitlab/__version__.py`.

For example in `gitlab/const.py` we have to know that
`gitlab.__version__` is a module and not the variable due to the
ordering of imports. But in most other usage `gitlab.__version__` is a
version string.

To reduce confusion make the name of the version file
`gitlab/_version.py`.

Create return type-hints for `RESTObject.get_id()` and
`RESTObject.encoded_id`. Previously was saying they return Any. Be
more precise in saying they can return either: None, str, or int.

Updated a few remaining usages of `self.id` to use `self.encoded_id`
as for the most part we shouldn't be using `self.id`

There are now only a few (4 lines of code) remaining uses of
`self.id`, most of which seem that they should stay that way.

chore(objects): use `self.encoded_id` where could be a string

Updated a few remaining usages of `self.id` to use `self.encoded_id`
where it could be a string value.