Set minimum required to numpy 1.16 (released January 2019) since latest sympy doesn't support earlier version.

Author: ericpre

.github/workflows/tests.yml

@@ -23,7 +23,7 @@ jobs:
           - os: ubuntu-latest
             PYTHON_VERSION: 3.6
             OLDEST_SUPPORTED_VERSION: true
-            DEPENDENCIES: matplotlib==2.2.3 numpy==1.15.4 scipy==1.1 imagecodecs==2019.12.3
+            DEPENDENCIES: matplotlib==2.2.3 numpy==1.16.0 scipy==1.1 imagecodecs==2019.12.3
             PIP_SELECTOR: '[all, tests]'
             LABEL: /oldest
           # test minimum requirement

setup.py

@@ -47,7 +47,7 @@
 
 install_req = ['scipy>=1.1',
                'matplotlib>=2.2.3',
-               'numpy>=1.15.4',
+               'numpy>=1.16.0',
                'traits>=4.5.0',
                'natsort',
                'requests',