Bump gds-sso to 9.3.0

This is to fix an issue with transfer ot bearer tokens.  See
https://github.com/alphagov/signonotron2/pull/245 for details.
Merge pull request #195 from alphagov/bump_gds-sso

Bump gds-sso to 9.3.0
Rename creds constant for clarity

Make it clear that the contained token is for asset-manager only, not
for any api clients.
Merge pull request #196 from alphagov/rename_creds_constant

Rename creds constant for clarity