Fix numpy py3.9 dep

Author: JacobHayes

pyproject.toml

@@ -3,4 +3,10 @@ requires-python = ">=3.6"
 
 [build-system]
 build-backend = "setuptools.build_meta"
-requires = ["numpy==1.19.0", "setuptools", "wheel"]
+# We need at least 1.19.0 for ABI compatibility, but otherwise prefer the oldest available wheel (eg: 1.19.3 for py3.9)
+requires = [
+    "numpy==1.19.0; python_version<'3.9'",
+    "oldest-supported-numpy",
+    "setuptools",
+    "wheel"
+]