CLN: Fix compile time warnings
- [x] passes `git diff upstream/master | flake8 --diff`

This commit suppresses these warnings

warning: comparison of constant -1 with expression\
of type 'PANDAS_DATETIMEUNIT' is always true\
[-Wtautological-constant-out-of-range-compare]

can u run all tests on Windows and report (if u can) otherwise I will
 Sorry I can not setup Windows environments soonly 😢 
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13607?src=pr) is **84.33%**

> Merging [#13607](https://codecov.io/gh/pydata/pandas/pull/13607?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13607   diff @@
==========================================
  Files           138        138          
  Lines         51100      51100          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43097      43097          
  Misses         8003       8003          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [2f7fdd0...e9eee1d](https://codecov.io/gh/pydata/pandas/compare/2f7fdd07eb0925016a28cf9ff324e351eac0c4df...e9eee1d4b31358d4b32dd5cb9b8ab40d1657d9e1?src=pr)
 ok @yui-knk this lgtm. and tests out on windows. (the warning only seems to appear on linux/osx), but that's ok!

pls add a whatsnew entry in bug fix for 0.19.0, reference this PR. ping when green.
 @jreback I added a whatsnew entry :)
 thanks @yui-knk 
 No problem 😄 
