Derive package version dynamically from git tag on publish

Use setuptools-scm to derive the package version from git tags at build
time. This removes the need to manually bump the version in a separate
commit before creating a release — the version is read directly from the
git tag (e.g. v1.3.0 -> 1.3.0).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jsoucheiron

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "pycfmodel"
-version = "1.2.0"
+dynamic = ["version"]
 readme = "README.md"
 description="A python model for Cloud Formation scripts"
 requires-python=">=3.9.0"
@@ -21,10 +21,12 @@ email = "security@skyscanner.net"
 
 [build-system]
 requires = [
-    "setuptools", "wheel"
+    "setuptools", "setuptools-scm", "wheel"
 ]
 build-backend = "setuptools.build_meta"
 
+[tool.setuptools_scm]
+
 [tool.setuptools.packages.find]
 include = ["pycfmodel*"]
 exclude = ["tests", "docs"]