Update testing/constraints-3.9.txt

Author: chalmerlowe

testing/constraints-3.9.txt

@@ -6,7 +6,7 @@
 # e.g., if setup.py has "foo >= 1.14.0, < 2.0.0dev",
 # Then this file should have foo==1.14.0
 db-dtypes==1.0.4
-numpy==1.18.1
+numpy==1.19.4
 pandas==1.1.4
 pyarrow==4.0.0
 pydata-google-auth==1.5.0