Present a better warning about no .rst doctesting on Python 3

Prior to this, you get this hunk of junk when testing on Python 3:

```
/tmp/astropy-test-fk1jae/lib.linux-x86_64-3.3/astropy/tests/pytest_plugins.py:252: UserWarning: Running doctests in .rst files is not yet supported on Python 3
  "Running doctests in .rst files is not yet supported on Python 3")
/tmp/astropy-test-fk1jae/lib.linux-x86_64-3.3/astropy/tests/pytest_plugins.py:252: UserWarning: Running doctests in .rst files is not yet supported on Python 3
  "Running doctests in .rst files is not yet supported on Python 3")
/tmp/astropy-test-fk1jae/lib.linux-x86_64-3.3/astropy/tests/pytest_plugins.py:252: UserWarning: Running doctests in .rst files is not yet supported on Python 3
  "Running doctests in .rst files is not yet supported on Python 3")
/tmp/astropy-test-fk1jae/lib.linux-x86_64-3.3/astropy/tests/pytest_plugins.py:252: UserWarning: Running doctests in .rst files is not yet supported on Python 3
  "Running doctests in .rst files is not yet supported on Python 3")
/tmp/astropy-test-fk1jae/lib.linux-x86_64-3.3/astropy/tests/pytest_plugins.py:252: UserWarning: Running doctests in .rst files is not yet supported on Python 3
  "Running doctests in .rst files is not yet supported on Python 3")
/tmp/astropy-test-fk1jae/lib.linux-x86_64-3.3/astropy/tests/pytest_plugins.py:252: UserWarning: Running doctests in .rst files is not yet supported on Python 3
  "Running doctests in .rst files is not yet supported on Python 3")
/tmp/astropy-test-fk1jae/lib.linux-x86_64-3.3/astropy/tests/pytest_plugins.py:252: UserWarning: Running doctests in .rst files is not yet supported on Python 3
  "Running doctests in .rst files is not yet supported on Python 3")
/tmp/astropy-test-fk1jae/lib.linux-x86_64-3.3/astropy/tests/pytest_plugins.py:252: UserWarning: Running doctests in .rst files is not yet supported on Python 3
  "Running doctests in .rst files is not yet supported on Python 3")```

Now it's part of the informational header:

```
Numpy: 1.8.0
Scipy: not available
Matplotlib: not available
h5py: not available
Running doctests in .rst files is not supported on Python 3.x
```