Use aiohttp for the HTTP requests instead of requests

[ViktorBarzin]

Is your feature request related to a problem? Please describe.
Nordigen client uses requests library which makes blocking http calls. This creates bottlenecks when fetching data for multiple requisition ids.

Describe the solution you'd like
Use aiohttp library which is asycnio-native. This will enable concurrent work using asyncio.gather which will dramatically speed up execution when working with multiple requistion ids.

The codebase is fairly small and I'm happy to make the change if the owners are happy with the idea.

[marcus302]

I would also be happy with this. If I implement this myself I'll share it back.