Scale ISO 8601 duration fractions by their digit count

[ckarnell]

The pure-Python ISO 8601 parser divides a fractional part by 10 whatever its length:

_days, _hours = _days.split(".")
days = int(_days)
hours = int(_hours) / 10 * HOURS_PER_DAY

So "1.25" gives int("25") / 10, which is 2.5 days instead of 0.25.

from pendulum.parsing.iso8601 import parse_iso8601

parse_iso8601("P1.25D").total_seconds()    # 302400, want 108000
parse_iso8601("PT1.25H").total_seconds()   # 12600,  want 4500
parse_iso8601("PT1.05M").total_seconds()   # 90,     want 63

Single-digit fractions are right, which is why P1.5D looks fine and hides it. The same / 10 appears for weeks, days, hours and minutes, so all four scale wrong together.

This is the fallback path, reached through PENDULUM_EXTENSIONS=0 or when the extension will not import, so the Rust parser isn't affected and gives the right answers throughout.

Dividing by 10 ** len(part) is the whole change. That's it.

The tests import parse_iso8601 directly, since parse goes through the extension and would not exercise this. P1.5D is in the table as a control: it passes either way, so the other four failing on revert is the fix and not the test. Suite is 1843 passing with PENDULUM_EXTENSIONS=0 and 1843 with the extension, up from 1838 both ways.