[Python] Error when submitting variables through Mailgun with Python

I'm trying to send mail through Mailgun with Python, in that instance I'm trying to push a variable alongside.

My code is the following:

requests.post("MailGun URL",
auth=("api", "api-key"),
data={
"from":"From_Mail",
"to":["To_Mail"],
"template":"template_name",
"subject":"Testing some things",
"h:X-Mailgun-Variables": {"firstname": "John"}
})


If I use the above without "X-Mailgun-Variables" I'm not experiencing any problems, but with the above I get the following error message:

"delivery-status": {
"code": 621,
"message": "",
"attempt-no": 1,
"description": "Failed to decode variables",
"session-seconds": 0
}


What am I doing wrong?

Thanks in advance,

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