python3: Use addtask statement instead of task dependencies

The externalsrc class deletes do_patch task which results with:
| ERROR: Task do_create_manifest in <PATH>/python3_3.8.2.bb depends upon
| non-existent task do_patch in <PATH>/python3_3.8.2.bb

Use addtask to define correct order to prevent this error, since addtask
mechanism accepts deleted tasks.

[YOCTO #14151]

(From OE-Core rev: a746d034)

Signed-off-by: Tomasz Dziendzielski <tomasz.dziendzielski@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 8ed8b81af60c7d9c7a1c614ad137408637bc43ed)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>