BUG: don't allow users to move from an interned string
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

I was thinking about code paths that give away borrowed references to strings and realized that interned strings can be accessed even when their refcount is 1. This is because the intern table holds a "weak ref" to the string. This allows the string to be garbage collected normally; however, if a user creates an interned string with exactly one reference and feeds it to move, you could later get a second reference to the **same** string object by calling intern with a copy of the old string. The data backing the old string may have been mutated so you will get very strange results. I am almost certain that no on has run into this since `move_into_mutable_buffer` was added but it doesn't hurt to add this quick check to prevent broken usage like this in the future.

This only affects python 2 because bytes objects cannot be interned in python 3.

I read somewhere that PY3 _only_ interns short strings. Is that not right?
 how did you find this issue (out of curiosity)? IOW, what broke?
 I am not sure what gets interned by default, but users may call `sys.intern` on any string to intern it I believe. This is not a concern for `move` in py3 because strings are unicode type, and this function only accepts bytes type. I found this by thinking about ways to mutate string objects with tools like `np.frombuffer` and thought that I could try to craft bad inputs to this which would give me a mutable string. Nothing broke, this was just the result of some experiments.
 Also, it looks like only one of the py3.4 builds failed:
`FAIL: test_downcast_limits (pandas.tools.tests.test_util.TestToNumeric)`

I am not sure how my change would have affected this, and only in the particular configuration. Does anyone have any ideas?
 ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14494?src=pr) is 85.28% (diff: 100%)

> Merging [#14494](https://codecov.io/gh/pandas-dev/pandas/pull/14494?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #14494   diff @@
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  Files           140        140          
  Lines         50693      50693          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43234      43234          
  Misses         7459       7459          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [1d6dbb4...ae8aa64](https://codecov.io/gh/pandas-dev/pandas/compare/1d6dbb41b26a39121ec8c4f19f5da78bb0ab4af7...ae8aa648c8a8c388af45dc1d8be6e11680dfc001?src=pr)
 this looks fine. wait till we merge #14631 then push again to force rebuilds.
 Tests are now passing after a rebase
 ping
 thanks!
 Thank you!
