BF: boost min cython to 0.23
closes #14699
closes #14508

prob need to change a few travis setups in ci
for this to work

 what is btw really the lowest version of cython which works in your setups and passes the tests? 0.23 here is somewhat arbitrary one higher than 0.21.1 where 14699 manifested ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14831?src=pr) is 85.26% (diff: 100%)
> Merging [#14831](https://codecov.io/gh/pandas-dev/pandas/pull/14831?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

```diff
@@             master     #14831   diff @@
==========================================
  Files           144        144          
  Lines         50980      50980          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43469      43469          
  Misses         7511       7511          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [0c82abe...e796e8b](https://codecov.io/gh/pandas-dev/pandas/compare/0c82abef8bc8fc7419a3b8823229cafd195595ac...e796e8bd1128fcef2d355e293de32430b27bebc0?src=pr) ```
ci/install_travis.sh:    pip install cython==0.19.1
ci/requirements-2.7.build:cython=0.19.1
ci/requirements-2.7_COMPAT.build:cython=0.19.1
ci/requirements-2.7_LOCALE.build:cython=0.19.1
``` @yarikoptic I think 0.23 is fine. if you can add a note on 0.20.0 whatsnew. also need to update ``install.rst`` I think (as well as change those files I listed above). interesting -- now 2nd env in travis fails due to failing to find pandas/index.c   any immediate clue why? @yarikoptic that build is still installing cython 0.19.1 (see the requirements-2.7_LOCALE.build file), so it acts like cython is not installed (I only thought we had a better error message for such cases) LGTM

@jreback  seem some files have no changes?

can you add a note in the whatsnew 0.20.0 (in addition to the one you have), about the development requireing a higher version.

can you see if anything needs to be changed in contributing.rst

otherwise lgtm. @jreback 

- "no changes" -- read between the lines ;)  git would not have a change registered if there were no changes -- while at it fixed some permissions
- whatsnew entry was added a while back... now had to resolve merge conflict already.  would appreciate a merge ;) thank you sir!