Add deprecation warnings for content moved to astropy-helpers

```
In [1]: import astropy.setup_helpers
WARNING: AstropyDeprecationWarning: The astropy.setup_helpers module has been deprecated, use astropy-helpers.setup_helpers instead [astropy.setup_helpers]

In [2]: import astropy.sphinx
WARNING: AstropyDeprecationWarning: The astropy.sphinx module has been deprecated, use astropy-helpers.sphinx instead [astropy.sphinx]

In [3]: import astropy.version_helpers
WARNING: AstropyDeprecationWarning: The astropy.version_helpers module has been deprecated, use astropy-helpers.version_helpers instead [astropy.version_helpers]

In [4]: from astropy.tests.helper import astropy_test

In [11]: from distutils.core import Distribution

In [12]: astropy_test(Distribution())
WARNING: AstropyDeprecationWarning: The astropy.tests.helper.astropy_test class has been deprecated, use astropy-helpers.astropy_test instead [astropy.tests.helper]
```