avoid pkg_resources import if it can (#44)

src/flatten_dict/__init__.py

@@ -1,7 +1,14 @@
-import pkg_resources
-
 from .flatten_dict import flatten, unflatten  # noqa: F401
 
 
-__all__ = ["flatten_dict", "reducer", "splitter"]
-__version__ = pkg_resources.get_distribution("flatten-dict").version
+__all__ = ["flatten", "unflatten", "splitter"]
+
+try:
+    from importlib.metadata import distribution
+except ImportError:
+    try:
+        from importlib_metadata import distribution
+    except ImportError:
+        from pkg_resources import get_distribution as distribution
+
+__version__ = distribution("flatten-dict").version