BLD: don't require cython on sdist install
- [x] closes #14204
- [x] tests not needed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

We could just make a cython a build-time requirement, but given we already create a sdists with `.c` files, probably makes sense to continue to support it.

I tested this locally, applying this patch on the latest source [release](https://pypi.python.org/packages/58/2c/62ba69f6cf16693fc341df084e8262ebb89f678ddc534164099fc7ddeb0b/pandas-0.19.0.zip#md5=6243f3abeaed035720bd77237bdade53) and it installed into a an environment without cython fine.

Thanks! Looks good to me, it's an easy fix to support installing without cython, so would keep doing that
 ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14475?src=pr) is 85.25% (diff: 100%)

> Merging [#14475](https://codecov.io/gh/pandas-dev/pandas/pull/14475?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #14475   diff @@
==========================================
  Files           140        140          
  Lines         50655      50655          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43185      43185          
  Misses         7470       7470          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [233d51d...8d93366](https://codecov.io/gh/pandas-dev/pandas/compare/233d51dd099f590379bd9144c40a61f6785b2b57...8d93366d381454ddad02142bb097a30e0cc5c98a?src=pr)
 thanks!
