build: Remove 'toml' extra from setuptools_scm build requirement (#517)

* The toml extra from setuptools-scm is now empty and exists only
  for legacy purposes and so can be removed.
* The modern setuptools-scm docs recommend a lower bound of
  setuptools v64 and setuptools-scm v8.
   - c.f. https://setuptools-scm.readthedocs.io/en/v8.1.0/

pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
 # Minimum requirements for the build system (setup.py) to execute.
-requires = ["setuptools>=42", "setuptools_scm[toml]>=3.4",]
+requires = ["setuptools>=64", "setuptools_scm>=8"]
 build-backend = "setuptools.build_meta"