Remove the extension from its own buildout.

Having isotoma.buildout.basicauth as extension in this buildout configuration
results in installing the latest released version.
Using the extension in its own buildout is not really necessary since we do
not access any private resources which would require authentication.
Add "keyring" to test-dependencies.

"keyring" is required for some tests, therefore it should also be installed.
pypirc: basicauth:credentials should override distutils:index-servers.

According to the current readme the basicauth:credentials option in ~/.pypirc
is used to list the servers used for authentication, but the previous
implementation did only respect distutils:index-servers.