Fix: 500 after applying coupon for Quince

[DmytroAlipov]

This is a backport from the master (already merged).

Description

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'

[openedx-webhooks]

Thanks for the pull request, @DmytroAlipov! Please note that it may take us up to several weeks or months to complete a review and merge your PR.

Feel free to add as much of the following information to the ticket as you can:

• supporting documentation
• Open edX discussion forum threads
• timeline information ("this must be merged by XX date", and why that is)
• partner information ("this is a course on edx.org")
• any other information that can help Product understand the context for the PR

All technical communication about the code itself will be done via the GitHub pull request interface. As a reminder, our process documentation is here.

Please let us know once your PR is ready for our review and all tests are green.

[cmltaWt0]

Verified the fix on a test Env.
Merging.

[openedx-webhooks]

@DmytroAlipov 🎉 Your pull request was merged! Please take a moment to answer a two question survey so we can improve your experience in the future.