Merge #1772

1772: Bump serde from 1.0.103 to 1.0.104 r=mergify[bot] a=dependabot-preview[bot]

Bumps [serde](https://github.com/serde-rs/serde) from 1.0.103 to 1.0.104.
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<summary>Release notes</summary>

*Sourced from [serde's releases](https://github.com/serde-rs/serde/releases).*

> ## v1.0.104
> - Revert stabilization of Serialize/Deserialize impls for `!` to account for [rust-lang/rust#67224](https://github-redirect.dependabot.com/rust-lang/rust/pull/67224)
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<summary>Commits</summary>

- [`234fbfd`](serde-rs/serde@234fbfd) Release 1.0.104
- [`b9909ce`](serde-rs/serde@b9909ce) Revert "Remove never_type feature gate"
- [`d540e72`](serde-rs/serde@d540e72) Format with rustfmt 2019-10-07
- [`13d0899`](serde-rs/serde@13d0899) Add missing import in de_enum expand test
- [`4fefa7a`](serde-rs/serde@4fefa7a) Format imports in expandtest inputs
- [`9048660`](serde-rs/serde@9048660) Update test suite to nightly-2019-11-26
- [`0b303c7`](serde-rs/serde@0b303c7) Mark tests ignored on emscripten rather than omitting
- [`c371563`](serde-rs/serde@c371563) Reduce nesting of macrotest test cases
- [`d17d8eb`](serde-rs/serde@d17d8eb) Rely on std::process::Command's path search
- [`bcaebdb`](serde-rs/serde@bcaebdb) Merge pull request [#1662](https://github-redirect.dependabot.com/serde-rs/serde/issues/1662) from eupn/use-macrotest
- Additional commits viewable in [compare view](serde-rs/serde@v1.0.103...v1.0.104)
</details>
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