Serialization Fix Release

dumpdata should now work in all contexts.

What's Changed

• Create basic base_manager when default_manager supplied by @bckohan in #816
• Use non-polymorphic managers for all invocations of dumpdata by @bckohan in #814
• docs: Add comprehensive fixture support documentation by @JohananOppongAmoateng in #791
• bump version, update changelog by @bckohan in #817

Full Changelog: v4.8.0...v4.9.0