Explicitly declare our build backend

This change more accurately reflects that ``setuptools`` is intended to
use the ``build_meta`` backend from a previous installation. This may
cause some problems with source builds if existing wheels for ``wheel``
and ``setuptools`` are not available. In that event, manually building a
source wheel is recommended.

pyproject.toml

@@ -1,5 +1,6 @@
 [build-system]
-requires = ["wheel"]
+requires = ["setuptools", "wheel"]
+build-backend="setuptools.build_meta"
 
 [tool.towncrier]
     package = "setuptools"