TST: Fix doctest failures - #10874

The RTD job only confirms that it can render into HTML without errors or warnings. It does not run any doctest. Hope this clarifies the matter!

Hmmm! Behavior change in numpy-dev . @mhvk , can you please advise?

122 Numpy array creation functions cannot be used to initialize Quantity
123 --------------------------------------------------------------------
124 Trying the following example will throw an UnitConversionError::
125 
126     >>> my_quantity = u.Quantity(1, u.m)
127     >>> np.full(10, my_quantity)  # doctest: +IGNORE_EXCEPTION_DETAIL
Expected:
    Traceback (most recent call last):
    ...
    UnitConversionError: 'm' (length) and '' (dimensionless) are not convertible
Got:
    array([1., 1., 1., 1., 1., 1., 1., 1., 1., 1.])
/home/travis/build/astropy/astropy/docs/known_issues.rst:127: DocTestFailure

Wow, that's interesting/weird... I guess little choice but to .. doctest-requires:: numpy<1.20 and adjust the text above, adding "on numpy before version 1.20 and ignore the unit in later versions:"

Great, thanks so much! Hopefully it will now all pass...

All green (even the numpy-dev job)!

Weirdly it doesn't cause a traceback on 1.16 on the LTS branch with 32bit 3.6, so I just go ahead and ignore the output as a workaround for that branch.

(I suppose I could remove the backport, but this is almost at the bottom of the pile and would be more painful that doing a quick workaround)