> ==6832==    at 0x4D87E9: PyUnicode_Decode (unicodeobject.c:3395)

This warning is a false alarm: the GCC builtin strcmp() function seems to read past the NUL byte. When recompiling Python with -fno-builtin GCC option, the warning is gone.

The valgrind warning can be reproduced using attached valgrind_strcmp_warn.c:

On Fedora 30 with:

* gcc (GCC) 9.2.1 20190827 (Red Hat 9.2.1-1)
* valgrind-3.15.0

GCC builtin strcmp() emits a false alarm in valgrind: it reads past the NUL
byte.

$ gcc -O3 valgrind_strcmp_warn.c -o valgrind_strcmp_warn -D NDEBUG=1 -g && valgrind ./valgrind_strcmp_warn
(...)
==29173== Conditional jump or move depends on uninitialised value(s)
==29173==    at 0x4011CB: PyUnicode_Decode.part.0 (valgrind_strcmp_warn.c:276)
==29173==    by 0x4898F42: (below main) (in /usr/lib64/libc-2.29.so)
==29173==
==29173== Conditional jump or move depends on uninitialised value(s)
==29173==    at 0x4011EE: PyUnicode_Decode.part.0 (valgrind_strcmp_warn.c:280)
==29173==    by 0x4898F42: (below main) (in /usr/lib64/libc-2.29.so)
==29173==
==29173== Conditional jump or move depends on uninitialised value(s)
==29173==    at 0x401221: PyUnicode_Decode.part.0 (valgrind_strcmp_warn.c:282)
==29173==    by 0x4898F42: (below main) (in /usr/lib64/libc-2.29.so)
(...)
==29173== ERROR SUMMARY: 3 errors from 3 contexts (suppressed: 0 from 0)

versus

$ gcc -O3 valgrind_strcmp_warn.c -o valgrind_strcmp_warn -D NDEBUG=1 -fno-builtin -g && valgrind ./valgrind_strcmp_warn
==29217== Memcheck, a memory error detector
(...)
==29217== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)