`isoformat` handles negative timezone offset incorrectly

[bkeryan]

The hightime.datetime implementation of isoformat expects the timezone to have a plus sign. When the timezone has a minus sign, it doesn't split the timezone before adding the femtoseconds and yoctoseconds, so you end up with a timezone in the middle of the string.

Steps to reproduce:

>>> import datetime as dt, hightime as ht
>>> dt.datetime(2025,1,1,tzinfo=dt.timezone(dt.timedelta(hours=-1))).isoformat()
'2025-01-01T00:00:00-01:00'
>>> ht.datetime(2025,1,1,tzinfo=dt.timezone(dt.timedelta(hours=-1))).isoformat()
'2025-01-01T00:00:00-01:00'
>>> ht.datetime(2025,1,1,yoctosecond=40,tzinfo=dt.timezone(dt.timedelta(hours=-1))).isoformat()
'2025-01-01T00:00:00-01:00.000000000000000000000040'

Expected output:

'2025-01-01T00:00:00.000000000000000000000040-01:00'

Note that the fix is not as easy as using minus instead of plus, because the string may contain other minus signs.