[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 Job ID Name Coverage
  Build
passed #139053
docker
build_linux_27

00:08:12

45.0%
passed #139054
docker
build_linux_36

00:08:11

45.0%
passed #139055
macosx
build_macosx_27

00:08:04

45.0%
passed #139056
macosx
build_macosx_36

00:07:47

45.0%
 
  Docker
passed #139057
docker-build
docker_linux_27

00:19:55

48.0%
passed #139058
docker-build
docker_linux_36

00:19:53

48.0%