add nutils.__version__ alias

In a follow-up patch `setup.py` will be replaced by a `pyproject.toml` using
Flit as build system. To prepare for this change, this patch adds
`nutils.__version__` as an alias for `nutils.version`, which is used by Flit to
determine the version of the package.

nutils/__init__.py

@@ -7,7 +7,7 @@
 assert sys.version_info >= (3, 5)
 assert LooseVersion(numpy.version.version) >= LooseVersion('1.16'), 'nutils requires numpy 1.16 or higher, got {}'.format(numpy.version.version)
 
-version = '8.0a0'
+__version__ = version = '8.0a0'
 version_name = None
 long_version = ('{} "{}"' if version_name else '{}').format(version, version_name)