prservice.py: fix do_package with newer Python in Ubuntu 20.04

* with Ubuntu 20.04 which is using python 3.8 I'm seeing a lot of errors like:
  ERROR: libxml2-2.9.10-r0 do_package: Can NOT get PRAUTO, exception No module named '_sysconfigdata'
  not sure what caused this from python 3.8, but this seems to work
* PRserv is enabled with:
  PRSERV_HOST = "localhost:0"

(From OE-Core rev: 4b26eaf7)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>