Initial pylint check is added. A LONG list of disabled checks is also
added. In the future we should work through the list and resolve the
errors or disable them on a more granular level.

Enable 'warn_redundant_casts'for mypy and resolve one issue.

docs: only use type annotations for documentation

chore: attempt to be more informative for missing attributes

Have code use constants from the gitlab.const module instead of from
the top-level gitlab module.

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.

A commonly reported issue from users on Gitter is that they get an
AttributeError for an attribute that should be present. This is often
caused due to the fact that they used the `list()` method to retrieve
the object and objects retrieved this way often only have a subset of
the full data.

Add more details in the AttributeError message that explains the
situation to users. This will hopefully allow them to resolve the
issue.

Update the FAQ in the docs to add a section discussing the issue.

Closes #1138