Add fractional second support to str{p,f}time

The ISO 8601 standard does not mandate any specific precision for
fractional seconds, so this accepts input of any length, ignoring the
part after the nanoseconds place. It may be more correct to round with
the tenths of nanoseconds digit, but then we'd have to deal with
carrying the round through the entire Tm struct (e.g. for a time like
Dec 31 11:59.999999999999).

%f is the format specifier that Python's datetime library uses for
0-padded microseconds so it seemed appropriate here.

cc #2350

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