for #25762: Add platform and python version info to user-agent

Adding the platform that the script is running on, and python version in order to be a little more consistent with the user-agent that any typical browser would send.

There's also server-side code that attempts to assign the platform for the current user. Since the API doesn't send any information that helps the server determine what platform it's running from, the server sets it to blank. This could possibly cause issues when doing things like matching against LocalStorage paths, etc. which require platform information.

Old default user agent string: `shotgun-json (3.0.17)`
New default user agent string: `shotgun-json (3.0.17); Python 2.6 (Mac)`