[schema] improve normalization of RQLExpressions
Parse and print back the expression instead of manipulating the string.
Among other benefits, it means we don't mangle embedded string constants
that contain commas or multiple spaces.
Closes #6694426
fromcubicweb.schemas.baseimportin_group,CWSourceSchemaConfig# copy __permissions__ to avoid modifying a shared dictionaryin_group.__permissions__=in_group.__permissions__.copy()in_group.__permissions__['read']=('managers',)cw_for_source=CWSourceSchemaConfig.get_relation('cw_for_source')cw_for_source.__permissions__={'read':('managers','users'),'add':('managers',),'delete':('managers',)}