Replace dateutil package with ciso8601

[seedofjoy]

Parsing datetime by dateutil is pretty slow. On my machine parsing 100 datetime's = 16ms, which its quite long.

ciso8601 is written as C module and 600x more faster than dateutil (https://github.com/closeio/ciso8601#benchmark)

What about replace dateutil with ciso8601? I can make MR

[anti-social]

You're welcome. My only opinion it should be an optional dependency.