chore: Use PEP 517 build system

[henryiii]

This makes the pyproject.toml file available for configuration (such as for #455).

Pros:

• Gives correct results if wheel is missing (normally a non-wheel build is made, which doesn't pre-cache all pyc files, etc).
• Gives correct results in an environment without setuptools (starting to become more common).
• Is not affected by the current Python environment's installs.
• No effect on legacy pips (pip 9 or less), they work as before.
• Enables using the file for other configuration.
• Likely will become the pip default in the future.

Cons:

• Can be slightly slower (1-2 seconds) when building from source due to setting up a temporary virtual environment. Besides "correctness is better than speed" for building, this is almost never activated, since this is a pure Python project and has wheels. Users installing from wheels do not even have the pyproject.toml file or setup.py file anyway.
• Causes pip to be really verbose when you add -v about trivial package searching.

Wrongly considered cons:

• This does not mean you have to access the internet when building from source. If a package is cached that matches the requirements (say, setuptools 43 is present), then it just uses that and doesn't need to access PyPI.
• For special situations, you can disable the build isolation; conda-build for example does this.