Add requirements.txt to downstream build (#401)

* Add requirements.txt to downstream build

This file is needed for execution environments.

* Add changelog fragment

changelogs/fragments/401-requirements_file.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+  - fix missing requirements.txt file in kubernetes.core (https://github.com/ansible-collections/community.kubernetes/pull/401).

utils/downstream.sh

@@ -34,6 +34,7 @@ f_prep()
         Makefile
         setup.cfg
         .yamllint
+        requirements.txt
     )
 
     # Directories to recursively copy downstream (relative repo root dir path)