refactor: derive __version__ from package metadata (single source)

Signed-off-by: disqualifier <dev@disqualifier.me>

src/mongo/__init__.py

@@ -1,7 +1,14 @@
+from importlib.metadata import PackageNotFoundError, version
+
 from .mongo import MongoDB, Mongo, init, instance
 
 __all__ = ["MongoDB", "Mongo", "init", "instance"]
 
+try:
+    __version__ = version("mongo")
+except PackageNotFoundError:
+    __version__ = "0.0.0+unknown"
+
 
 def __getattr__(name: str):
     """proxy bare attribute access to the default instance (PEP 562); needs