use python3 for some dependencies

tasks/prepare.yml

@@ -8,7 +8,9 @@
 
     - name: Install required dependencies
       pip:
-        name: "{{ __drone_pip_dependencies }}"
+        name: "{{ item.name }}"
+        executable: "{{ item.exec | default(omit) }}"
         state: present
+      loop: "{{ __drone_pip_dependencies }}"
   become: True
   become_user: root

vars/main.yml

@@ -4,5 +4,6 @@ __droneci_default_no_proxy:
   - droneagent
 
 __drone_pip_dependencies:
-  - docker
-  - docker-compose
+  - name: docker
+  - name: docker-compose
+    exec: pip3