Fix 500 after applying coupon for Palm

[DmytroAlipov]

Description

This is a backport from the master branch.

Steps to Reproduce:

• Login by the user with staff rights
• During purchase click on the link "Click here to purchase multiple seats in this course"
• Confirm and pay
• Activate the course with one of the coupons sent to your email
• Open /dashboard page

The issue was that the date-time also included milliseconds, which is not acceptable for converting the date and time into the required format.

ValueError: time data '2033-06-25T11:40:12.267999Z' does not match format '%Y-%m-%dT%H:%M:%SZ'

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[mphilbrick211]

Hi @colinbrash! Would you or another member of the @openedx/revenue-squad (purchase) be able to review and merge this for us? Thanks!

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