Update dependency postcss to v7.0.20

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
postcss (source)	devDependencies	patch	7.0.18 -> 7.0.20

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Release Notes

postcss/postcss

### [`v7.0.20`](https://togithub.com/postcss/postcss/blob/master/CHANGELOG.md#​7020) [Compare Source](https://togithub.com/postcss/postcss/compare/7.0.19...7.0.20) - Allow to pass PostCSS’s nodes in `nodes` property to node constructor. ### [`v7.0.19`](https://togithub.com/postcss/postcss/blob/master/CHANGELOG.md#​7019) [Compare Source](https://togithub.com/postcss/postcss/compare/7.0.18...7.0.19) - Fix passing `nodes` property to node constructor.

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