incorrect formatting of negative numbers

[mbarkhau]

With Python

In [1]: "{:,.2f}%".format(-200)
Out[1]: '-200.00%'

With the python-format lib

format('{:,.2f}%', -200)
"-,200.00%"

[mbarkhau]

The issue may be fixed on this fork: https://github.com/NextTuesday/python-format