Create a custom `warnings.warn` wrapper that will walk the stack trace
to find the first frame outside of the `gitlab/` path to print the
warning against. This will make it easier for users to find where in
their code the error is generated from

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.

Fix the import ordering using isort.

https://pycqa.github.io/isort/

F401: Module imported but unused

https://www.flake8rules.com/rules/F401.html

Initially when extracting out the gitlab/client.py code we tried to
remove this but functional tests failed.

Later we fixed the functional test that was failing, so now remove the
unneeded import.

Move the classes Gitlab and GitlabList from gitlab/__init__.py to the
newly created gitlab/client.py file.

Update one test case that was depending on requests being defined in
gitlab/__init__.py