post.py

import urllib.request
import os
import json

token = os.getenv('GITHUB_TOKEN')
data = {
    'text': '**hello** __world__',
}
headers = {
    'Content-Type': 'application/json',
    'Authorization': f'token {token}'
}

url = 'https://api.github.com/markdown'
req = urllib.request.Request(url, json.dumps(data).encode(), headers)
with urllib.request.urlopen(req) as res:
    print(res.read().decode('utf8'))