network checks: report 301 redirects (only)

[mgorny]

I've looked into the network check code, and I have two suggestions:

1. FetchablesUrlCheck should also report redirects,
2. Both checks should report only 301 (permanent) redirects. Reporting 302 is simply incorrect.

[radhermit]

I was waiting to do this until I ported it to use the requests library instead of urllib since it's much easier to do using that.

[radhermit]

Unfortunately requests doesn't support ftp:// links though so urllib support will probably have to stick around anyway for that.