refactor: derive __version__ from package metadata (single source)

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

src/envelope_crypto/__init__.py

@@ -1,3 +1,5 @@
+from importlib.metadata import version, PackageNotFoundError
+
 from .envelope_crypto import (
     EnvelopeCrypto,
     DocumentCrypto,
@@ -12,6 +14,11 @@ from .envelope_crypto import (
     fingerprint_data,
 )
 
+try:
+    __version__ = version("envelope_crypto")
+except PackageNotFoundError:
+    __version__ = "0.0.0+unknown"
+
 __all__ = [
     "EnvelopeCrypto",
     "DocumentCrypto",
@@ -24,4 +31,5 @@ __all__ = [
     "decrypt_document",
     "decrypt_dict",
     "fingerprint_data",
+    "__version__",
 ]