Remove vendored versioneer

Remove vendored versioneer.py and load versioneer as a build dependency instead. Fixes problems with PEP-517 editable installs (c.f. https://github.com/python-versioneer/python-versioneer/issues/192).

src/pystencils/__init__.py

@@ -36,7 +36,5 @@ __all__ = ['Field', 'FieldType', 'fields',
            'fd',
            'stencil']
 
-from ._version import get_versions
-
-__version__ = get_versions()['version']
-del get_versions
+from . import _version
+__version__ = _version.get_versions()['version']