Remove ImportError catching from `from contextlib import redirect_stdout`

[KevinHock]

Now that we're python 3+, we can remove the following: https://github.com/Yelp/detect-secrets-server/blob/cf25d3e95a97daaf236799cb4e38940396834c2c/detect_secrets_server/adhoc/github/webhook.py#L10-L19

(Still reusing some code from this project :D)

[P0intMaN]

Hello Developer,

I am new to open source contribution. I would like this to be my first contribution towards this repository. I hope this issue is still open and do I have to take permission from moderators before contributing?

Thanks!