Version 4.0.0 has been tested, so no breaking changes from 3.x.x

fixes side-effect of `requirementslib` not specifying `chardet` as dep
sarugaku/requirementslib#296 is still open

User will not need to install `chardet` manually to use this package

Adding `chardet` to Pipfile seems to have changed the markers for `mypy`
Compatibility for 3.7 broken; `six-stub` package is needed; can be added
`chardet` moved to `dev-packages`

Locked with `pipenv lock --keep-outdated`
Less diff in Pipfile.lock vs b442e71 and 72eb526
No other dep changes needed