MNT: Remove fsl/__init__.py - previously we used pkgutil-style namespace

packages to preserve compatibility with python 2. But py2-compat is no longer
a goal, so we can now use native namespace packages.

fsl/__init__.py

-#!/usr/bin/env python
-#
-# __init__.py - The fslpy library.
-#
-# Author: Paul McCarthy <pauldmccarthy@gmail.com>
-#
-"""The :mod:`fsl` package is a library which contains convenience classes
-and functions for use by FSL python tools. It is broadly split into the
-following sub-packages:
-
-.. autosummary::
-
-   fsl.data
-   fsl.utils
-   fsl.scripts
-   fsl.transform
-   fsl.version
-   fsl.wrappers
-
-.. note:: The ``fsl`` namespace is a ``pkgutil``-style *namespace package* -
-          it can be used across different projects - see
-          https://packaging.python.org/guides/packaging-namespace-packages/
-          for details.
-"""
-
-__path__ = __import__('pkgutil').extend_path(__path__, __name__)  # noqa