TST/MAINT: Update python/numpy version in test matrix

The test matrix was last updated in 2019. Using versions of
NumPy and Python that have reached their end of life. This commit
bumps the versions used to those that are still within their
lifetime.

Author: Kai-Striega

.github/workflows/pythonpackage.yml

@@ -9,8 +9,8 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.6, 3.7]
-        numpy-version: [1.15, 1.16, 1.17]
+        python-version: ["3.7", "3.8", "3.9", "3.10"]
+        numpy-version: ["1.20", "1.21", "1.22", "1.23"]
 
     steps:
     - uses: actions/checkout@v1