downgrade to numpy 1.18.1

(cherry picked from commit f60baa6)

Author: schmide

requirements/env_climada.yml

@@ -16,7 +16,7 @@ dependencies:
   - nbformat=5.0.7
   - netcdf4=1.5.4
   - numba=0.51.2
-  - numpy=1.19.1
+  - numpy=1.18.1
   - matplotlib=3.2.2
   - matplotlib-base=3.2.2
   - pandas=1.0.5