MAINT: Pin NumPy for Python 3.9

Author: bashtage

pyproject.toml

@@ -5,8 +5,9 @@ requires = [
   "numpy==1.15.4; python_version=='3.6'",
   "numpy==1.15.4; python_version=='3.7'",
   "numpy==1.17.5; python_version=='3.8'",
-  "numpy; python_version>='3.9'",
-  "Cython>=0.29.21"]
+  "numpy==1.19.5; python_version=='3.9'",
+  "numpy; python_version>'3.9'",
+  "Cython>=0.29.22"]
 
 [tool.black]
 target-version = ['py37', 'py38']