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

See: https://docs.gitlab.com/ee/api/personal_access_tokens.html

The variable ALLOWED_KEYSET_ENDPOINTS was added in commit
f86ef3bbdb5bffa1348a802e62b281d3f31d33ad.

Then most of that commit was removed in commit
e71fe16b47835aa4db2834e98c7ffc6bdec36723, but ALLOWED_KEYSET_ENDPOINTS
was missed.

httplib is a Python 2 library. It was renamed to http.client in Python
3.

https://docs.python.org/2.7/library/httplib.html