Change minimum Python version to 3.10

Since we started using type hints, including the `|` operator to indicate a union of multiple possible arguments, Python 3.10 is now the new minimum required version.

setup.cfg

@@ -16,7 +16,7 @@ classifiers =
 
 [options]
 packages = find:
-python_requires = >=3.9
+python_requires = >=3.10
 install_requires =
     numpy>=1.21
     scipy>=1.11