Update dependency pdm-backend to v2.3.1 (#73)

This PR contains the following updates:

| Package | Type | Update | Change | Pending |
|---|---|---|---|---|
| [pdm-backend](https://togithub.com/pdm-project/pdm-backend) |
build-system.requires | patch | `==2.3.0` -> `==2.3.1` | `2.3.2` |

---

### Release Notes

<details>
<summary>pdm-project/pdm-backend (pdm-backend)</summary>

###
[`v2.3.1`](https://togithub.com/pdm-project/pdm-backend/releases/tag/2.3.1)

[Compare
Source](https://togithub.com/pdm-project/pdm-backend/compare/2.3.0...2.3.1)

#####    🐞 Bug Fixes

- Handle unexpected hg repo when parse scm version  -  by
[@&#8203;huxuan](https://togithub.com/huxuan) in
[https://github.com/pdm-project/pdm-backend/issues/237](https://togithub.com/pdm-project/pdm-backend/issues/237)
[<samp>(d6fec)</samp>](https://togithub.com/pdm-project/pdm-backend/commit/d6feccf)
- Treat PDM_BUILD_SCM_VERSION empty string as unset  -  by
[@&#8203;ryukinix](https://togithub.com/ryukinix) in
[https://github.com/pdm-project/pdm-backend/issues/238](https://togithub.com/pdm-project/pdm-backend/issues/238)
[<samp>(596f1)</samp>](https://togithub.com/pdm-project/pdm-backend/commit/596f1dc)
- Honor the current working directory when importing pdm_build.py Close
[#&#8203;245](https://togithub.com/pdm-project/pdm-backend/issues/245)
 -  by [@&#8203;frostming](https://togithub.com/frostming) in
[https://github.com/pdm-project/pdm-backend/issues/245](https://togithub.com/pdm-project/pdm-backend/issues/245)
[<samp>(ba98c)</samp>](https://togithub.com/pdm-project/pdm-backend/commit/ba98c85)
- Errors when using the `editables` backend with the `includes` field.
Fixes
[#&#8203;242](https://togithub.com/pdm-project/pdm-backend/issues/242)
 -  by [@&#8203;frostming](https://togithub.com/frostming) in
[https://github.com/pdm-project/pdm-backend/issues/242](https://togithub.com/pdm-project/pdm-backend/issues/242)
[<samp>(59396)</samp>](https://togithub.com/pdm-project/pdm-backend/commit/59396e3)
- Vendor editables  -  by
[@&#8203;frostming](https://togithub.com/frostming)
[<samp>(95da7)</samp>](https://togithub.com/pdm-project/pdm-backend/commit/95da73a)

#####     [View changes on
GitHub](https://togithub.com/pdm-project/pdm-backend/compare/2.3.0...2.3.1)

</details>

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Co-authored-by: erichesse <[email protected]>

pyproject.toml

@@ -93,5 +93,5 @@ known-first-party = ["mex", "tests"]
 convention = "google"
 
 [build-system]
-requires = ["pdm-backend==2.3.0"]
+requires = ["pdm-backend==2.3.1"]
 build-backend = "pdm.backend"