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`.