Drop support for Python 3.5 & breaking performance improvements

[thibaudcolas]

• Removes support for Python 3.5, following the lead from black / Django / Wagtail
• Adds black’s install and validation in CI
• Switch to Python 3.6+ f-strings wherever possible, and Python 3.6+ variable type annotations
• Remove HTML attributes alphabetical sorting of default string engine (breaking change)
• Stop sorting inline styles alphabetically (breaking change)
• Disable single and double quotes escaping outside of attributes for string engine (breaking change)

---

• [x] Stay on point and keep it small so it can be easily reviewed. For example, try to apply any general refactoring separately outside of the PR.
• [x] Consider adding unit tests, especially for bug fixes. If you don't, tell us why.
• [x] All new and existing tests pass, with 100% test coverage (make test-coverage)
• [x] Linting passes (make lint)
• [x] Consider updating documentation. If you don't, tell us why.
• [x] List the environments / platforms in which you tested your changes.

Thanks for contributing to this project!