chore(deps): update dependency pymdown-extensions to v10.21 (#8)

This PR contains the following updates:

| Package | Change |
[Age](https://docs.renovatebot.com/merge-confidence/) |
[Confidence](https://docs.renovatebot.com/merge-confidence/) |
|---|---|---|---|
|
[pymdown-extensions](https://redirect.github.com/facelessuser/pymdown-extensions)
| `==10.20.1` → `==10.21` |
![age](https://developer.mend.io/api/mc/badges/age/pypi/pymdown-extensions/10.21?slim=true)
|
![confidence](https://developer.mend.io/api/mc/badges/confidence/pypi/pymdown-extensions/10.20.1/10.21?slim=true)
|

---

### Release Notes

<details>
<summary>facelessuser/pymdown-extensions (pymdown-extensions)</summary>

###
[`v10.21`](https://redirect.github.com/facelessuser/pymdown-extensions/releases/tag/10.21)

#### 10.21

- **NEW**: Caption: Add support for specifying not only IDs but classes
and arbitrary attributes. Initial work by
  [@&#8203;joapuiib](https://redirect.github.com/joapuiib).
- **FIX**: MagicLink: Fix a matching pattern for Bitbucket repo.

</details>

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requirements.txt

@@ -20,7 +20,7 @@ paginate==0.5.7
 pathspec==1.0.4
 platformdirs==4.4.0
 Pygments==2.19.2
-pymdown-extensions==10.20.1
+pymdown-extensions==10.21
 python-dateutil==2.9.0.post0
 PyYAML==6.0.3
 pyyaml_env_tag==1.1