Bump is-svg and postcss-svgo (nortonandrews#19)

Removes [is-svg](https://github.com/sindresorhus/is-svg). It's no longer used after updating ancestor dependency [postcss-svgo](https://github.com/cssnano/cssnano). These dependencies need to be updated together.


Removes `is-svg`

Updates `postcss-svgo` from 4.0.2 to 4.0.3
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](cssnano/cssnano@4.0.2...4.0.3)

---
updated-dependencies:
- dependency-name: is-svg
  dependency-type: indirect
- dependency-name: postcss-svgo
  dependency-type: indirect
...

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