Use importlib.metadata for version, single source of truth

Remove hardcoded version string from __init__.py and use
importlib.metadata.version() instead. Version is now defined
only in pyproject.toml.

Closes #22

Author: dannywillems

l9format/__init__.py

@@ -1,4 +1,6 @@
-__version__ = "1.4.0"
+from importlib.metadata import version
+
+__version__ = version("l9format")
 
 from l9format.l9format import (
     Certificate,