Add __version__ attribute to boltons package

Uses importlib.metadata to read the version from the installed package
metadata, falling back to "unknown" when running from a source checkout
without installing.

This allows users to do:
    import boltons
    print(boltons.__version__)

Author: bysiber

boltons/__init__.py

@@ -0,0 +1,7 @@
+from importlib.metadata import version, PackageNotFoundError
+
+try:
+    __version__ = version("boltons")
+except PackageNotFoundError:
+    # Package is not installed (e.g. running from source checkout)
+    __version__ = "unknown"