Merge pull request #597 from bsipocz/MAINT_fix_minimum_np

CI: bump oldest numpy version in CI

Author: bsipocz

tox.ini

@@ -42,7 +42,7 @@ deps =
     oldestdeps: astropy==4.1
     # We set a suitably old numpy along with an old astropy, no need to pick up
     # deprecations and errors due to their unmatching versions
-    oldestdeps: numpy==1.16
+    oldestdeps: numpy==1.19
 
     online: pytest-rerunfailures