Set meaningful User Agent

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Currently, the user agent is the default "python-requests" (python client); however, it would be useful to have something unique (+ version), which makes debugging at the Backend easier.

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headers = {'Authorization': 'Bearer {}'.format(token_text['access_token'])}

should be replaced by:

headers = {
  'Authorization': 'Bearer {}'.format(token_text['access_token']),
  'User-Agent': 'Sen2Cube.at ArcPy Toolbox v0-1-0'
}

(Optionally with a non-hardcoded version number)