Previously we strictly followed the RFC3339 format when parsing datetime
and time objects from strings. We now support a few common ISO8601
compatible relaxations:
- A `:` isn't required as part of the timezone component in both
  datetime and time strings (`2022-01-02T03:04:05.678+0102` and
  `2022-01-02T03:04:05.678+01:02` are treated the same).
- A ` ` may be used instead of `T`/`t` as a separator between date and
  time components when parsing datetime strings.
When encoding datetime/time objects we still strictly follow RFC3339.
This eases integrating msgspec with other systems that don't strictly
follow RFC3339.