Respect the pypirc basicauth:credentials option as described in the readme

The readme shows an example of how to define a list of authenticated servers, using the `basicauth:credentials` option:
```
[basicauth]
credentials =
    github
...
````

The current implementation did not read this option at all.
I think the `basicauth:credentials` option should take precedence over `distutils:index-servers` so that we can:

1. differentiate between downloads and uploads
2. use servers for downloads which we would not like to use for uploads *
3. use different credentials for uploads and downloads

*: for example zest.releaser asks to upload a dist for every index-server configured. Having there "github" for example might be annoying.


*Other changes in this pull-request*
- Removed the `isotoma.buildout.basicauth` from the `extensions` of its own buildout: this is not really necessary for running the tests and it results in downloading the latest release, since `develop = .` is not yet active.
- Added `keyring` to the tests-requires, since there are relevant tests.


Thanks for this extensions, it helps a lot :wink: