Fix Error Message Related to ISO 8601 format for `creationInfo` `created` field

[jspeed-meyers]

When a user provides a SPDX document with a time created field that has a fractional second component of an ISO 8601 datetime, tools-python returns an error message similar to this:

Invalid created value '2023-03-17T19:44:10.259Z' must be date in ISO 8601 format.

After discussion in issue #527 and spdx/ntia-conformance-checker#95, I believe that tools-python does not in fact strictly follow ISO 8601, which allows fractional seconds, and instead follows the SPDX spec, which does not allow fractional seconds.

To help users follow the format expected by the SPDX spec, @zvr suggested this error message instead:

Invalid created value '2023-03-17T19:44:10.259Z' is not in "YYYY-MM-DDThh:mm:ssZ" format.

I'm glad to put in a PR if the maintainers are amenable to this change.

[armintaenzertng]

Hi @jspeed-meyers, thanks for the thorough investigation of this issue! By all means, please go ahead and open a PR for this if you like! :)