Remove stray require from lib/index.js

[thomasjm]

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To fix #3854.

This adds a fixup step to the build, so the proper version string gets inserted into lib/index.js.

For example, at the current version, the file is now

// Copyright (c) Jupyter Development Team.
// Distributed under the terms of the Modified BSD License.
export * from './utils';
export * from './version';
export * from './widget_link';
...
export * from './widget_upload';
export const version = "5.0.7";
//# sourceMappingURL=index.js.map

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:point_left: Launch a binder notebook on branch codedownio/ipywidgets/remove-require