Lookup Sygnal version using the correct name

We passed it a module name [1] instead of the "distribution name"(?)

Broken in 0ced8bd, so Sygnal 0.5.0.

[1]: https://docs.python.org/3/reference/import.html#name__

sygnal/__init__.py

@@ -16,7 +16,7 @@
 from importlib.metadata import PackageNotFoundError, version
 
 try:
-    __version__ = version(__name__)
+    __version__ = version("matrix-sygnal")
 except PackageNotFoundError:
     # package is not installed
     pass