Missing `const` keyword

[fulopkovacs]

https://github.com/Backblaze/gists/blob/b5a73fef3e4d3ae14121decc1b364008acc18515/b2AuthorizeCfWorker.py#L69-L82

The modRequest variable in line 76 is in the global scope, which shouldn't break the functionality of the worker, but in my opinion it'd be safer to make it local by putting a const keyword before it.