Fix build warnings
Cleans up a whole bunch. Remaining offenders are:
- Parsers.pyx and tokenizer. Signed/unsigned hell, made worse by python/cython/c mixing.
- interval.pyx, not touched because large PR in progress in #27100.
- np_datetime.c and np_datetime_strings.c. These are linked into 15+ cython modules as extra object files, but they '#include <datetime.h>`, which expects only standalone modules, who properly init to do it. Kinda painful to fix, though it means each function is compiled and kept in memory 15 times.
