refactor: derive __version__ from package metadata (single source)

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

src/aiowebhooks/__init__.py

@@ -5,10 +5,15 @@ WebhookResult and never raises on a send failure. see README for usage. the
 [discord] extra adds DiscordWebhook (aiowebhooks.discord).
 """
 
+from importlib.metadata import PackageNotFoundError, version
+
 from .errors import NoUrlsError, WebhookError
 from .result import WebhookResult
 from .sender import Webhook
 
 __all__ = ["Webhook", "WebhookResult", "WebhookError", "NoUrlsError"]
 
-__version__ = "0.1.8"
+try:
+    __version__ = version("aiowebhooks")
+except PackageNotFoundError:
+    __version__ = "0.0.0+unknown"