Object of type datetime is not JSON serializable

To solve an object of type datetime that is not json serializable, you have to convert the Python datetime into a string that matches the very specific format accepted by Salesforce. Here's what I do in CumulusCI:

    def salesforce_from_datetime(d): """Create a Salesforce-style ISO8601 string from a Python datetime""" return d.strftime("%Y-%m-%dT%H:%M:%S.{}+0000").format( str(d.microsecond)[:3].ljust(3, "0") )

That does the ISO8601 formatting but also converts Python's microseconds into the milliseconds that the Salesforce API expects, with exactly three digits.