iStart by declaring the build backend

All packaging docs show the build-system table declared first:
https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#a-full-example
https://packaging.python.org/en/latest/specifications/pyproject-toml/

That's probably customary, not a requirement, but let's follow the trend.

Author: DimitriPapadopoulos

pyproject.toml

@@ -1,3 +1,7 @@
+[build-system]
+build-backend = "setuptools.build_meta"
+requires = ["setuptools>=77.0.2", "setuptools-scm>=7"]
+
 [project]
 authors = [{ name = "xarray Developers", email = "xarray@googlegroups.com" }]
 classifiers = [
@@ -95,10 +99,6 @@ source-code = "https://github.com/pydata/xarray"
 [project.entry-points."xarray.chunkmanagers"]
 dask = "xarray.namedarray.daskmanager:DaskManager"
 
-[build-system]
-build-backend = "setuptools.build_meta"
-requires = ["setuptools>=77.0.2", "setuptools-scm>=7"]
-
 [tool.setuptools.packages.find]
 include = ["xarray*"]