[FIX] base: ir.model.fields, write, use tools.ustr instead of str so it handle if the string is already unicode

Writing special chars in the field label resulted in a traceback (write only, no problem with creation)
[RVT] Reverting rev. 5117, as a different but equivalent fix for lp:1231900 is committed upstream
[FIX] base: ir.model.fields, write, use tools.ustr instead of str so it handle if the string is already unicode

Writing special chars in the field label resulted in a traceback (write only, no problem with creation)
[FIX] safe_eval: allow STORE_FAST and DELETE_FAST opcodes, used e.g. for comprehension expressions
[FIX] safe_eval: allow STORE_FAST and DELETE_FAST opcodes, used e.g. for comprehension expressions