Add mechanism to retry if API times out

[troyax]

Added utilities to do http requests and perform retries when the API takes long to respond.

Features

• Performs retry on add to cart if 5 secs already passed by
• Performs retry on get stock if 10 secs already passed by

Caveats

• If the API responds with timeout before aiohttp throws timeout error itself it will be handled as an error and do not perform a retry. Can be avoided if the response status 504 is handled too.
• After some quick testing, seems that the average time in order for aiohttp to throw is about 10-15 secs, more than that and the API will throw and not aiohttp

[philippnormann]

Hey @troyax, thanks for the suggestion! The latest version has a working retry mechanism for the API. I'm closing this for now, but feel free to open a new PR if you have any suggestions on improving the current retry mechanism.