make github api calls in parallel

[chris48s]

In f5fcf29 I added pagination, but if we need to query more repos than the batch size, the API calls will be made in series.

I think the case where we need to actually make lots of API calls is relatively rare. Most python applications don't have hundreds and hundreds of dependencies.

That said, there is no dependency between the requests so I could switch from requests to httpx and fire all the requests off in a asyncio.gather() to improve performance in the case where we have to make multiple round-trips to GitHub.