Fix missing AstropyDeprecationWarning import

[saimn]

Missing AstropyDeprecationWarning in astropy.io.fits.column.

In [33]: coldefs = fits.ColDefs([c1], tbtype='BinTableHDU')
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-33-844778bb414c> in <module>()
----> 1 coldefs = fits.ColDefs([c1], tbtype='BinTableHDU')

/home/simon/lib/astropy/astropy/io/fits/column.py in __new__(cls, input, tbtype, ascii)
   1156                 'inferred from the formats of the supplied columns.  Use the '
   1157                 '``ascii=True`` argument to ensure that ASCII table columns '
-> 1158                 'are used.', AstropyDeprecationWarning)
   1159         else:
   1160             tbtype = 'BinTableHDU'  # The old default

NameError: global name 'AstropyDeprecationWarning' is not defined

I guess this tbtype arguments is not widely used ;-). The docstring says it was deprecated in Astropy 0.4, so maybe it is time to remove it ?

[pllim]

According to APE2, "Furthermore, when possible, a backwards incompatible change should generate an AstropyDeprecationWarning for at least one feature release version before actually making the change permanent, to allow codes to transition to the new interface."

That means 0.4 was several feature releases ago, so I am for removing the deprecated feature.

[astrofrog]

Let's go ahead and merge this fix for now, and it would be great if someone could go through and look for all such deprecation warnings that could be removed (I think if they were added before 1.0 they can definitely be removed) - @saimn @pllim, feel free to do this if you like! :)

[saimn]

Yep, I will take a look later at deprecated stuff, there are many in the fits module.