Add support for async

[blokje]

We use an async approach to all of our API's. As such I took the liberty on creating an aiohttp equivalent of pyvat. Is this something to consider including into your code? If so, some more work has to be done in order to clean things up and remove redundant code. Otherwise we will just keep using it in our internal fork.

At this moment it works mostly as an drop-in replacement, except for the need to use async functions of course ;)

import asyncio
from pprint import pprint

async def aio_main():
    import pyvat.aio as pyvat
    x = await pyvat.check_vat_number('001786519B01', 'NL')
    pprint(vars(x))

def main():
    import pyvat
    x = pyvat.check_vat_number('001786519B01', 'NL')
    pprint(vars(x))

main()
asyncio.run(aio_main())