This allows accessing the elements by value, i.e.:

import gitlab.const
gitlab.const.AccessLevel(20)

As shown here, Admin access is set to 60:
https://docs.gitlab.com/ee/api/protected_branches.html#protected-branches-api

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`.

We are planning on adding enumerated constants into gitlab/const.py,
but if we do that than they will end up being added to the top-level
gitlab namespace. We really want to get users to start using
`gitlab.const.` to access the constant values in the future.

Add the currently defined constants to a list that should not change.
Use a module level __getattr__ function so that we can deprecate
access to the top-level constants.

Add a unit test which verifies we generate a warning when accessing
the top-level constants.

BREAKING CHANGE: remove deprecated constants defined in
gitlab.v4.objects, and use only gitlab.const module