Move package version from `hcloud.__version__.VERSION` to `hcloud.__version__`

[jooola]

Feature Request

Is your feature request related to a problem? Please describe.

The package version is currently stored in hcloud.__version__.VERSION: in the hcloud.__version__ module inside a VERSION constant.

To simplify access to the version, I propose to move and rename the VERSION constant (or reexport it) inside the hcloud/__init__.py file, so we can directly access it with hcloud.__version__.

In addition, once we only support python 3.8, we could also use the importlib.metadata standard library to fetch the version from the package metadata.