These are encoded to ISO8601 duration strings. Only `'D'`, `'H'`, `'M'`,
and `'S'` units are supported (no relative units like `Y` or `M`
(month)).

If `strict=False` we also support decoding int/float values (or str
representations of these values) to `timedelta` types. Note that a
`double` cannot represent a timedelta with microsecond precision if
the value is > ~270 years. For true roundtrip support we recommend
serializing timedeltas as ISO 8601 strings.