chore: release v0.3.1 (#928)

## 🤖 New release
* `hugr`: 0.3.0 -> 0.3.1 (✓ API compatible changes)

<details><summary><i><b>Changelog</b></i></summary><p>

<blockquote>

## 0.3.1 (2024-04-23)

### Features

- `new` methods for leaf ops
([#940](#940))
- `CustomOp::extension` utility function
([#951](#951))
- Encoder metadata in serialized hugr
([#955](#955))

### Testing

- Add a test of instantiating an extension set
([#939](#939))
</blockquote>


</p></details>

---
This PR was generated with
[release-plz](https://github.com/MarcoIeni/release-plz/).

Signed-off-by: Agustin Borgna <agustin.borgna@quantinuum.com>
Co-authored-by: Agustin Borgna <agustin.borgna@quantinuum.com>

hugr/CHANGELOG.md

@@ -1,5 +1,18 @@
 # Changelog
 
+## 0.3.1 (2024-04-23)
+
+### Features
+
+- `new` methods for leaf ops ([#940](https://github.com/CQCL/hugr/pull/940))
+- `CustomOp::extension` utility function ([#951](https://github.com/CQCL/hugr/pull/951))
+- Encoder metadata in serialized hugr ([#955](https://github.com/CQCL/hugr/pull/955))
+
+### Testing
+
+- Add a test of instantiating an extension set ([#939](https://github.com/CQCL/hugr/pull/939))
+
+
 ## 0.3.0 (2024-04-15)
 
 ### Main changes

hugr/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "hugr"
-version = "0.3.0"
+version = "0.3.1"
 edition = { workspace = true }
 rust-version = { workspace = true }