refactor datetime parsing and validation

[vincentsarago]

ref #132 (comment)

A while ago stac-fastapi moved to ciso8601 and then to pyiso8601 (stac-utils/stac-fastapi#448) but I think now it's safe to use ciso8601 for datetime parsing and validation (at least if using ~=2.3, developmentseed/tipg#39 (comment))

[codecov-commenter]

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##             main     #133   +/-   ##
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  Hits            ?      578           
  Misses          ?       19           
  Partials        ?        0           

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