refactor: derive __version__ from package metadata (single source)

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

src/psql/__init__.py

@@ -1,3 +1,10 @@
+from importlib.metadata import PackageNotFoundError, version
+
 from .psql import PsqlDB
 
 __all__ = ["PsqlDB"]
+
+try:
+    __version__ = version("psql")
+except PackageNotFoundError:
+    __version__ = "0.0.0+unknown"