I reopen the issue, the PyUnicode_Decode warning is not fixed yet.

vstinner@apu$ echo|PYTHONMALLOC=malloc valgrind ./python Lib/tokenize.py
==6832== Memcheck, a memory error detector
==6832== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==6832== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==6832== Command: ./python Lib/tokenize.py
==6832== 
==6832== Conditional jump or move depends on uninitialised value(s)
==6832==    at 0x4D87E9: PyUnicode_Decode (unicodeobject.c:3395)
==6832==    by 0x4D8E6E: PyUnicode_FromEncodedObject (unicodeobject.c:3268)
==6832==    by 0x4D9283: unicode_new (unicodeobject.c:15125)
==6832==    by 0x49271B: type_call (typeobject.c:969)
==6832==    by 0x440E97: _PyObject_MakeTpCall (call.c:167)
==6832==    by 0x42D6D3: _PyObject_Vectorcall (abstract.h:104)
==6832==    by 0x42D6D3: call_function (ceval.c:4984)
==6832==    by 0x42D6D3: _PyEval_EvalFrameDefault (ceval.c:3496)
==6832==    by 0x4216CA: function_code_fastcall (call.c:292)
==6832==    by 0x4411B5: _PyObject_FastCallDict (call.c:109)
==6832==    by 0x441413: _PyObject_Call_Prepend (call.c:447)
==6832==    by 0x497320: slot_tp_init (typeobject.c:6772)
==6832==    by 0x492767: type_call (typeobject.c:989)
==6832==    by 0x440E97: _PyObject_MakeTpCall (call.c:167)
==6832== 
1,0-1,1:            NL             '\n'           
2,0-2,0:            ENDMARKER      ''             
==6832== 
==6832== HEAP SUMMARY:
==6832==     in use at exit: 1,179,135 bytes in 8,467 blocks
==6832==   total heap usage: 93,677 allocs, 85,210 frees, 12,109,969 bytes allocated
==6832== 
==6832== LEAK SUMMARY:
==6832==    definitely lost: 0 bytes in 0 blocks
==6832==    indirectly lost: 0 bytes in 0 blocks
==6832==      possibly lost: 454,934 bytes in 1,972 blocks
==6832==    still reachable: 724,201 bytes in 6,495 blocks
==6832==         suppressed: 0 bytes in 0 blocks
==6832== Rerun with --leak-check=full to see details of leaked memory
==6832== 
==6832== Use --track-origins=yes to see where uninitialised values come from
==6832== For lists of detected and suppressed errors, rerun with: -s
==6832== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)