Update serde_yaml requirement from 0.8 to 0.9

[dependabot]

Updates the requirements on serde_yaml to permit the latest version.

Release notes

Sourced from serde_yaml's releases.

0.9.0

API documentation: https://docs.rs/serde_yaml/0.9

Highlights

• The serde_yaml::Value enum gains a Tagged variant which represents the deserialization of YAML's !Tag syntax. Tagged scalars, sequences, and mappings are all supported.

• An empty YAML input (or document containing only comments) will deserialize successfully to an empty map, empty sequence, or Serde struct as long as the struct has only optional fields. Previously this would error.

• A new .apply_merge() method on Value implements YAML's << merge key convention.

• The Debug representation of serde_yaml::Value has gotten vastly better (dtolnay/serde-yaml#287).

• Deserialization of borrowed strings now works.

  #[derive(Deserialize, Debug)]
  struct Struct<'a> {
      borrowed: &'a str,
  }
  let yaml = "borrowed: 'kölcsönzött'\n";
  let value: Struct = serde_yaml::from_str(yaml)?;
  println!("{:#?}", value);

• Value's and Mapping's methods get and get_mut have been generalized to support a &str argument, as opposed to requiring you to allocate and construct a Value::String for indexing into another existing Value.

• Mapping exposes more APIs that have become conventional on map data structures, such as .keys(), .values(), .into_keys(), .into_values(), .values_mut(), and .retain(|k, v| …).

Breaking changes

• Serialization no longer produces leading ---\n on the serialized output. You can prepend this yourself if your use case demands it.

• Serialization of enum variants is now based on YAML's !Tag syntax, rather than JSON-style singleton maps.

  #[derive(Serialize, Deserialize)]
  enum Enum {
      Newtype(usize),
      Tuple(usize, usize, usize),
      Struct { x: f64, y: f64 },
  }

  - !Newtype 1
  - !Tuple [0, 0, 0]
  - !Struct {x: 1.0, y: 2.0}

... (truncated)

Commits

• 4167a95 Release 0.9.0
• f2ef142 Update readme to cover enums
• 040f0a0 Merge pull request #291 from dtolnay/tovec
• 2c0c5f2 Remove serde_yaml::to_vec
• d9f7442 Merge pull request #290 from dtolnay/seed
• ec88cd7 Remove seed module and functions
• 33caf06 Document Value::Tagged
• 895ddf9 Document enum representations
• 728df48 Improve more Debug representations
• 92a9bdb Support unit variant nested inside another enum
• Additional commits viewable in compare view

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