download: add netrc file support

Like for URL that contain credentials, we still skip auth challenge
because it's faster and:
- we only support one auth scheme (basic)
- netrc provides no way to specify realms, which seem anyway to be
  less and less used (https://stackoverflow.com/q/69303610 reports
  that recent browsers don't display them anymore)

See merge request !25
2 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Buildout.UnitTest-Master.Python2

00:08:31

passed SlapOS.Buildout.UnitTest-Master.Python3

00:12:51