Fix ordinal() suffix for negative numbers

[itzzdev09]

Calling ordinal() on a negative integer gives the wrong suffix:

>>> ordinal(-1)
'-1th'
>>> ordinal(-9)
'-9st'
>>> ordinal(-21)
'-21th'

The suffix is chosen from value % 10 and value % 100, but Python's modulo of a negative number counts up from the next lower multiple (-9 % 10 == 1, -1 % 10 == 9), so the digit used for the lookup is wrong for every negative value.

Computing the digit from abs(value) fixes it while leaving non-negative values, the 11/12/13 special case, and the non-finite / non-numeric paths unchanged:

>>> ordinal(-1)
'-1st'
>>> ordinal(-9)
'-9th'
>>> ordinal(-21)
'-21st'

Added the negative cases to test_ordinal; the full test_number.py suite passes.

[hugovk]

Please see questions on duplicate #321.