Bump trio from 0.20.0 to 0.21.0 in /dependencies/default (#368)

Bumps [trio](https://github.com/python-trio/trio) from 0.20.0 to 0.21.0.
- [Release notes](https://github.com/python-trio/trio/releases)
- [Commits](python-trio/trio@v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: trio
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

dependencies/default/constraints.txt

@@ -18,7 +18,7 @@ pytest-trio==0.7.0
 sniffio==1.2.0
 sortedcontainers==2.4.0
 tomli==2.0.1
-trio==0.20.0
+trio==0.21.0
 typed-ast==1.5.4
 typing_extensions==4.2.0
 zipp==3.8.0