Update toml_edit requirement from 0.19.0 to 0.21.0 (#225)

Updates the requirements on [toml_edit](https://github.com/toml-rs/toml) to permit the latest version.
- [Commits](toml-rs/toml@v0.19.0...v0.21.0)

---
updated-dependencies:
- dependency-name: toml_edit
  dependency-type: direct:production
...

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rstest_test/Cargo.toml

@@ -15,7 +15,7 @@ version = "0.11.0"
 
 [dependencies]
 regex = "1.5.6"
-toml_edit = "0.19.0"
+toml_edit = "0.21.0"
 
 [dev-dependencies]
 lazy_static = "1.4.0"