fix numpy version for python 3.9

Author: jdkent

setup.cfg

@@ -24,7 +24,8 @@ classifiers =
 [options]
 python_requires = >= 3.8
 install_requires =
-    numpy>=1.8.0
+    numpy>=1.8.0,<2.0; python_version == "3.9" and extra == 'stan'
+    numpy>=1.8.0; python_version != "3.9" or extra != 'stan'
     pandas
     scipy
     sympy