Wheel-inspect

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- Upgraded `wheel_inspect.SCHEMA` from JSON Schema draft 4 to draft 7
- Don't require directory entries in wheels to be listed in `RECORD`

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- Include `pyproject.toml` in `MANIFEST.in`, thereby making it possible to
build from sdist

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- `.derived.keywords` is now sorted and duplicate-free

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- `"buildver"` is now `None`/`null` instead of the empty string when there is
no build tag in the wheel's filename
- Added a `parse_wheel_filename()` function for parsing a wheel filename into
its components
- Validation of `RECORD` files is now done directly by `wheel-inspect` instead
of with `distlib` in order to achieve more descriptive error messages

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Initial release

This project's code was previously part of
[Wheelodex](https://github.com/jwodder/wheelodex).