refactor: derive __version__ from package metadata (single source)

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

src/envelope_authorizer/__init__.py

@@ -1 +1,6 @@
-__version__ = "0.1.7"
+from importlib.metadata import version, PackageNotFoundError
+
+try:
+    __version__ = version("envelope_authorizer")
+except PackageNotFoundError:
+    __version__ = "0.0.0+unknown"