Use oldest-supported-numpy in pyproject.toml

Although we need some newer features of NumPy at runtime, we
can safely build against the older headers. I was not able
to detect any performance regressions from doing so either.

See dwavesystems/dwave-ocean-sdk#208

pyproject.toml

@@ -3,7 +3,7 @@ requires = [
     "setuptools>=46.4.0",
     "wheel",
     "Cython>=0.29.21,<3.0",
-    'numpy~=1.21.0',
+    'oldest-supported-numpy',
 ]
 build-backend = "setuptools.build_meta"