Version 3.7.3 Release

[carltongibson]

• Create pull request for release notes based on the 3.7.3 milestone.
• Ensure the pull request increments the version to 3.7.3 in restframework/__init__.py.
• Ensure that release date is included in pull request.
• Merge the release pull request.
• Push the package to PyPI with ./setup.py publish.
• Tag the release, with git tag -a 3.7.3 -m 'version 3.7.3'; git push --tags.
• Make a release announcement on the discussion group.
• Make a release announcement on twitter.
• Close the milestone on GitHub.

[xordoquy]

Thanks @carltongibson

[carltongibson]

Thank you! And @rpkilby from for the speedy work.