GitHub backend seems to regularly issue 403s

[changlinli]

I feel like I've seen this a couple of times, but I'm getting the following:

Maybe some sort of rate limiting by GitHub? I imagine it's probably transient since so many projects have GitHub backends, but it might be worth looking into so GitHub doesn't block Anitya.

[changlinli]

Hmmm... okay that one had other issues (namely the version check URL is incorrect and yet it seems version checks worked anyway which is odd). Here's an example of one with a correct version check URL

[Zlopez]

This is definitely related to Github ratelimit. I have few feature requests (#589, #733) already created that should help with this.

[Zlopez]

From https://developer.github.com/v3/#rate-limiting I noticed that 403 is returned when the limit is already reached, which probably happens a lot.
We need to handle 403 on GitHub backend as RateLimitException instead of AnityaPluginException.