bpo-39646: Remove compile warnings in unicodeobject.c

[shihai1991]

https://bugs.python.org/issue39646

[codecov]

Codecov Report

Merging #18519 into master will increase coverage by 3.73%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master   #18519       +/-   ##
===========================================
+ Coverage   79.46%   83.20%    +3.73%     
===========================================
  Files         384     1571     +1187     
  Lines      169290   414730   +245440     
  Branches        0    44456    +44456     
===========================================
+ Hits       134529   345062   +210533     
- Misses      34761    60021    +25260     
- Partials        0     9647     +9647     

Impacted Files	Coverage Δ
Objects/stringlib/transmogrify.h
Parser/acceler.c
Objects/stringlib/localeutil.h
Objects/stringlib/unicode_format.h
Modules/_curses_panel.c
Modules/_decimal/libmpdec/transpose.h
Python/clinic/_warnings.c.h
Modules/_heapqmodule.c
Modules/_testmultiphase.c
Modules/_threadmodule.c
... and 1944 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f632736...30bab85. Read the comment docs.

[shihai1991]

cc @vstinner
Due to https://github.com/python/cpython/pull/18397/files

[vstinner]

I changed the bpo number in the commit message to: https://bugs.python.org/issue39500