Deprecated datetime module function fix

[lexe11]

Module

mail_tracking

Describe the bug

datetime.utcnow() & datetime.utcfromtimestamp() are deprecated and I changed them to datetime.now(timezone.utc) & datetime.fromtimestamp(int(timestamp), timezone.utc) respectively.

Expected behavior
Should fix an error.

Additional context
This module doesn't rely on SendGrid webhook events which makes email tracking very limited.