TimeDelta serialization precision

Hi there!

I just found quite strange behaviour of TimeDelta field serialization

from marshmallow.fields import TimeDelta
from datetime import timedelta

td_field = TimeDelta(precision="milliseconds")

obj = dict()
obj["td_field"] = timedelta(milliseconds=345)

print(td_field.serialize("td_field", obj))

Output of this snippet is 344, but it seems that 345 is correct.

Looks like a rounding issue here: https://github.com/marshmallow-code/marshmallow/blob/dev/src/marshmallow/fields.py#L1474

[deckar01]

This seems like a better option than going back to a long if chain that explicitly multiplies and adds the integer values together.

I assumed the introduction of additional time resolutions in #691 corresponded to timedelta also splitting the times across these units internally, but it actually doesn’t. The intermediate units get packed into the original 3 integers (days, seconds, microseconds). Cpython’s timedelta has a private method named _to_microseconds() with a simple formula that we can borrow.

[yarsanich]

@deckar01 thanks for fixing this!