[CLN] [BLD] Fix many compiler warnings
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](https://codecov.io/gh/pandas-dev/pandas/pull/22013?src=pr&el=h1) Report
> Merging [#22013](https://codecov.io/gh/pandas-dev/pandas/pull/22013?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/67b627717db2b0d6c1b3699d9384cc433c0b9af4?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22013/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/22013?src=pr&el=tree)

```diff
@@           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)` | :arrow_up: |
| #single | `42.17% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22013?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22013?src=pr&el=footer). Last update [67b6277...c19e7e5](https://codecov.io/gh/pandas-dev/pandas/pull/22013?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 https://travis-ci.org/pandas-dev/pandas/jobs/406697012

was this just worse before? there are still many unsigned compare with signed warnings. > 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. ok let's hope this and your other PR fixes more warnings :>