refactor: derive __version__ from package metadata (single source)

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

src/aioweb/__init__.py

@@ -1,7 +1,14 @@
+from importlib.metadata import version, PackageNotFoundError
+
 from .session import ExtendedSession, DEFAULT_ATTEMPTS, RETRY_STATUSES
 from .responses import Response, FailureResponse, AiowebError, aiowebResponse
 from .preview import RequestPreview
 
+try:
+    __version__ = version("aioweb")
+except PackageNotFoundError:
+    __version__ = "0.0.0+unknown"
+
 __all__ = [
     "ExtendedSession",
     "Response",