[common] Fix striginfy method

With Python 3 map is a generator function which is not serializable.

The fix is to make it a list.

Fixes #34
6 jobs for fix_stringify in 28 minutes and 7 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build_linux_27 #139053
docker

00:08:12

45.0%
passed build_linux_36 #139054
docker

00:08:11

45.0%
passed build_macosx_27 #139055
macosx

00:08:04

45.0%
passed build_macosx_36 #139056
macosx

00:07:47

45.0%
 
  Docker
passed docker_linux_27 #139057
docker-build

00:19:55

48.0%
passed docker_linux_36 #139058
docker-build

00:19:53

48.0%