Implements format builtin (sync of #760)

[martica]

This syncs #760 against the current master branch and updates the error messages and doc strings to match the current Python versions.

PR Checklist:

• [ ] All new features have been tested
• [x] All new features have been documented
• [x] I have read the CONTRIBUTING.md file
• [x] I will abide by the code of conduct

[danyeaw]

Thanks @lielfr and @martica!