[CLN] [BLD] Fix many compiler warnings

[jbrockmendel]

Checked in OSX in 2.7 and 3.7, on Ubuntu on 2.7 and 3.5, this fixes just about all the warnings that are fixable at my pay-grade.

The ones that are left:

• Ubiquitous NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION --> I expect there is a compiler flag that can be set in setup.py to silence this (and only this) warning, haven't figured it out.
• In Py3.7:

pandas/_libs/src/ujson/python/objToJSON.c:430:15: warning: initializing 'char *' with an expression of type 'const char *' discards qualifiers
      [-Wincompatible-pointer-types-discards-qualifiers]
        char *data = PyUnicode_AsUTF8AndSize(obj, &len);
              ^      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
pandas/_libs/src/ujson/python/objToJSON.c:436:14: warning: 'PyUnicode_EncodeUTF8' is deprecated [-Wdeprecated-declarations]
    newObj = PyUnicode_EncodeUTF8(PyUnicode_AS_UNICODE(obj),
             ^
/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/include/python3.7m/unicodeobject.h:1324:7: note: 'PyUnicode_EncodeUTF8' has been
      explicitly marked deprecated here
    ) Py_DEPRECATED(3.3);
      ^
/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/include/python3.7m/pyport.h:493:54: note: expanded from macro 'Py_DEPRECATED'
#define Py_DEPRECATED(VERSION_UNUSED) __attribute__((__deprecated__))
                                                     ^
3 warnings generated.

• On Linux:

In file included from pandas/_libs/src/datetime/np_datetime.h:21:0,
                 from pandas/_libs/src/datetime/np_datetime_strings.c:33:
/usr/include/python3.5m/datetime.h:191:25: warning: ‘PyDateTimeAPI’ defined but not used [-Wunused-variable]
 static PyDateTime_CAPI *PyDateTimeAPI = NULL;

• Also on Linux a bunch of "this may be unitialized" warnings.

[codecov]

Codecov Report

Merging #22013 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #22013   +/-   ##
=======================================
  Coverage   91.99%   91.99%           
=======================================
  Files         167      167           
  Lines       50578    50578           
=======================================
  Hits        46530    46530           
  Misses       4048     4048

Flag	Coverage Δ
#multiple	90.4% <ø> (ø)	⬆️
#single	42.17% <ø> (ø)	⬆️

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[jreback]

https://travis-ci.org/pandas-dev/pandas/jobs/406697012

was this just worse before? there are still many unsigned compare with signed warnings.

[jreback]

see comments

[jbrockmendel]

was this just worse before? there are still many unsigned compare with signed warnings.

Yes. The first commit here caused test failures (that I didn't get locally), so I reverted fixes in interval.pyx.

[jreback]

ok let's hope this and your other PR fixes more warnings :>