import tls_client
client = tls_client.Session()
print(client.post('https://httpbin.org/post', json={'a':'b'}).json()
{'args': {}, 'data': '{"a": "b"}', 'files': {}, 'form': {}, 'headers': {'Accept': '/', 'Accept-Encoding': 'gzip, deflate, br', 'Content-Length': '10', 'Content-Type': 'application/json', 'Host': 'httpbin.org', 'User-Agent': 'tls-client/1.0.1', 'X-Amzn-Trace-Id': 'Root=1-65f57b99-62f659ac717afad079b6f9f6'}, 'json': {'a': 'b'}, 'origin': '24.6.214.76', 'url': 'https://httpbin.org/post'}
print(client.post('https://httpbin.org/post', data={'a':'b'}).json() # content-type show be application/x-www-form-urlencoded
{'args': {}, 'data': 'a=b', 'files': {}, 'form': {}, 'headers': {'Accept': '/', 'Accept-Encoding': 'gzip, deflate, br', 'Content-Length': '3', 'Content-Type': 'application/json', 'Host': 'httpbin.org', 'User-Agent': 'tls-client/1.0.1', 'X-Amzn-Trace-Id': 'Root=1-65f57ba4-5caf4a7701cfc5eb735aac2d'}, 'json': None, 'origin': '24.6.214.76', 'url': 'https://httpbin.org/post'}

I already give a pull request in #116 to fix the problem