Fetch project version number dynamically

This ensures the build tool gets the correct version number from
`pyargus.VERSION`, rather than keeping the number in two places.

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "argus-api-client"
-version = "0.5.0"
+dynamic = ["version"]
 description = "A Python API client library for the Argus alert aggregator server"
 readme = "README.md"
 authors = [{name="Sikt - Kunnskapssektorens Tjenesteleverandør", email="kontakt@sikt.no"}]
@@ -20,6 +20,9 @@ dependencies = [
     "iso8601",
 ]
 
+[tool.setuptools.dynamic]
+version = {attr = "pyargus.VERSION"}
+
 [tool.setuptools.packages.find]
 where = ["src"]
 exclude = ["tests*"]