Build warnings re const-correctness with Python nightly (3.7a)

[encukou]

https://travis-ci.org/encukou/py3c/jobs/197263815

gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -g -fstack-protector --param=ssp-buffer-size=4 -Wformat -Werror=format-security -fPIC -I../include -I/opt/python/3.7-dev/include/python3.7m -c test_py3c.c -o build/temp.linux-x86_64-3.7/test_py3c.o -Werror -Wall
test_py3c.c: In function ‘str_asstring_check’:
test_py3c.c:46:14: error: initialization discards ‘const’ qualifier from pointer target type [-Werror]
test_py3c.c: In function ‘str_asutf8_check’:
test_py3c.c:86:14: error: initialization discards ‘const’ qualifier from pointer target type [-Werror]
test_py3c.c: In function ‘str_asutf8andsize_check’:
test_py3c.c:93:14: error: initialization discards ‘const’ qualifier from pointer target type [-Werror]
cc1: all warnings being treated as errors

[encukou]

@Traceur759, if you'd like a side quest, here's a possible one.