Update dependency sphinxcontrib-htmlhelp to <=2.1.0 (#32)

This PR contains the following updates:

| Package | Change | [Age](https://docs.renovatebot.com/merge-confidence/) | [Confidence](https://docs.renovatebot.com/merge-confidence/) |
|---|---|---|---|
| [sphinxcontrib-htmlhelp](https://redirect.github.com/sphinx-doc/sphinxcontrib-htmlhelp) ([changelog](https://redirect.github.com/sphinx-doc/sphinxcontrib-htmlhelp/blob/master/CHANGES.rst)) | `<=2.0.1` → `<=2.1.0` | ![age](https://developer.mend.io/api/mc/badges/age/pypi/sphinxcontrib-htmlhelp/2.1.0?slim=true) | ![confidence](https://developer.mend.io/api/mc/badges/confidence/pypi/sphinxcontrib-htmlhelp/2.0.1/2.1.0?slim=true) |

---

### Release Notes

<details>
<summary>sphinx-doc/sphinxcontrib-htmlhelp (sphinxcontrib-htmlhelp)</summary>

### [`v2.1.0`](https://redirect.github.com/sphinx-doc/sphinxcontrib-htmlhelp/blob/HEAD/CHANGES.rst#Release-210-2024-07-28)

[Compare Source](https://redirect.github.com/sphinx-doc/sphinxcontrib-htmlhelp/compare/2.0.6...2.1.0)

\==========================

- Adopt Ruff
- Tighten MyPy settings
- Update GitHub actions versions
- Escape HTML entities

### [`v2.0.6`](https://redirect.github.com/sphinx-doc/sphinxcontrib-htmlhelp/blob/HEAD/CHANGES.rst#Release-206-2024-07-20)

[Compare Source](https://redirect.github.com/sphinx-doc/sphinxcontrib-htmlhelp/compare/2.0.5...2.0.6)

\==========================

- Fix tests for Sphinx 7.4 and later.

### [`v2.0.5`](https://redirect.github.com/sphinx-doc/sphinxcontrib-htmlhelp/blob/HEAD/CHANGES.rst#Release-205-2024-01-13)

[Compare Source](https://redirect.github.com/sphinx-doc/sphinxcontrib-htmlhelp/compare/2.0.4...2.0.5)

\==========================

- Remove Sphinx as a required dependency, as circular dependencies may cause
  failure with package managers that expect a directed acyclic graph (DAG)
  of dependencies.

### [`v2.0.4`](https://redirect.github.com/sphinx-doc/sphinxcontrib-htmlhelp/blob/HEAD/CHANGES.rst#Release-204-2023-08-14)

[Compare Source](https://redirect.github.com/sphinx-doc/sphinxcontrib-htmlhelp/compare/2.0.3...2.0.4)

\==========================

- Use `os.PathLike` over `pathlib.Path`

### [`v2.0.3`](https://redirect.github.com/sphinx-doc/sphinxcontrib-htmlhelp/blob/HEAD/CHANGES.rst#Release-203-2023-08-09)

[Compare Source](https://redirect.github.com/sphinx-doc/sphinxcontrib-htmlhelp/compare/2.0.2...2.0.3)

\==========================

- Fix tests for Sphinx 7.1 and below

### [`v2.0.2`](https://redirect.github.com/sphinx-doc/sphinxcontrib-htmlhelp/blob/HEAD/CHANGES.rst#Release-202-2023-08-07)

[Compare Source](https://redirect.github.com/sphinx-doc/sphinxcontrib-htmlhelp/compare/2.0.1...2.0.2)

\==========================

- Drop support for Python 3.8
- Raise minimum required Sphinx version to 5.0

</details>

---

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Author: renovate[bot]

sphinx-plugin/pyproject.toml

@@ -32,7 +32,7 @@ dependencies = [
     "sphinx<5.0.0",
     "sphinxcontrib-applehelp<=2.0.0",
     "sphinxcontrib-devhelp<=1.0.6",
-    "sphinxcontrib-htmlhelp<=2.0.1",
+    "sphinxcontrib-htmlhelp<=2.1.0",
     "sphinxcontrib-serializinghtml<=1.1.10",
     "sphinxcontrib-qthelp<=1.0.8",
     "standard-imghdr; python_version >= '3.13'",