added __version__ attribute (read from _version.py)

Author: ChrisBarker-NOAA

xarray_subset_grid/__init__.py

@@ -6,3 +6,9 @@
 from .selector import Selector
 
 __all__ = ['Selector']
+
+try:
+    from ._version import __version__
+except ImportError:
+    __version__ = "version_unknown"
+