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]

Signed-off-by: Tomasz Dziendzielski <tomasz.dziendzielski@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit a746d034)
Signed-off-by: Steve Sakoman <steve@sakoman.com>