User-Agent for HackerRank

[kmyk]

ref #626 (comment)

Now HackerRank seems to be have banned User-Agents which contain python-requests as a substring. We use python-requests/x.y.z (+https://github.com/kmyk/online-judge-tools) as UA, and banned.
By the way, a string online-judge-tools is not banned. So it seems that we are just embroiled, and we should change the UA.

logs

https://github.com/kmyk/online-judge-tools/blob/7a8c94af9813dfd839ddf3c35c128c8fff12e90d/onlinejudge/_implementation/utils.py#L76

>>> requests.get('https://www.hackerrank.com/rest/contests/hourrank-30/challenges/video-conference/download_testcases')
<Response [403]>
>>> requests.get('https://www.hackerrank.com/rest/contests/hourrank-30/challenges/video-conference/download_testcases', headers={'User-Agent': 'curl/7.58.0'})
<Response [200]>