v0.4.12: allow human-readable date in ub.presigns()

feat:

• allow human-readable date in ub.presigns():

import unibox as ub

uri = "s3://bucket-external/dataset/dataset_qft/moody_qft_danbooru.json"
signed = ub.presigns(uri, expiration="1y")  # format: https://github.com/xolox/python-humanfriendly
signed