Actually reduce min required numpy version to 1.16

Fixes a mistake in c13ef75, where I tried this already but only changed
the version requirements for development, not the ones that end up in
the final python wheels.

pyproject.toml

@@ -14,7 +14,7 @@ classifiers = [
   "Intended Audience :: Science/Research",
 ]
 dependencies = [
-  "numpy~=1.19",
+  "numpy~=1.16",
 ]
 maintainers = [
   {name = "Robert Bamler", email = "robert.bamler@uni-tuebingen.de"},