Message 389762 - Python tracker

Current behavior:

>>> format(1234.1234, '_f')
'1_234.123400'
>>> format(1234.1234, ',f')
'1,234.123400'

New behavior:
>>> format(1234.1234, ',._f')
'1,234.123_400'
>>> format(1234.1234, '_._f')
'1_234.123_400'
>>> format(1234.1234, '._f')
'1234.123_400'
>>> format(1234.1234, '._4f')
'1234.123_4'
>>> format(1234.1234, '.f')  # still not allowed
'1234.123_4'
>>> format(1234.1234, '_.f')  # still not allowed