Build(deps): Bump thewalrus from 0.20.0 to 0.21.0

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

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

poetry.lock

@@ -60,7 +60,7 @@ scipy = "^1.11.4"
 setuptools = "*"
 sky130 = {git = "https://github.com/daquintero/skywater130.git", branch = "dario/patch_for_rladder_gplugins"}
 sky130-hdl21 = {version = "^6.0.0"}
-thewalrus = "^0.20.0"
+thewalrus = ">=0.20,<0.22"
 vlsirtools = "^6.0.0"
 xarray = "^2023.12.0"