When used as a library, current date is fixed at import time and not updated

[jennifer-richards]

When xml2rfc is imported and used as a library, it fixes the current date at import time (via datetime.date.today()) and does not update this. If the process using the library is long-lived, this causes files to be assigned incorrect dates.

Some care is needed in fixing this. When xml2rfc is used as a standalone program, this behavior may protect against date skew if run near midnight. (Just a guess)

This is probably causing ietf-tools/datatracker#3944