BUG: Remove locale conversion from Stata file date
Prevent locale from affecting Stata file date creation, which must
be en_US.

xref #13856

 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

Does not close any issue since this was only identified as part of #13856.  

Passes all tests (not just slow) when locale is `zh_CN.utf8`.  This config is not enabled by default. https://travis-ci.org/jreback/pandas/jobs/194872508

oddly this didn't fail on py3.  ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15208?src=pr) is 85.99% (diff: 100%)


> No coverage report found for **master** at 9309eba.

> Powered by [Codecov](https://codecov.io?src=pr). Last update [9309eba...a23a92f](https://codecov.io/gh/pandas-dev/pandas/compare/9309ebae91ca580d54a7d3d8d2b9db7df5e0d72b...a23a92f3eba48fa85a2bb680382c751521708927?src=pr) can you rebase and push again, just to make sure this passes? ping on green. @jreback Green - one test fails in another module that has the same problem due to locales.

https://travis-ci.org/pandas-dev/pandas/jobs/194970098

 thanks for the fix @bashtage 