Bump phonopy from 2.20.0 to 2.21.0 (#1301)

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Bumps [phonopy](https://phonopy.github.io/phonopy/) from 2.20.0 to
2.21.0.


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tests/requirements-covalent.txt

@@ -1,3 +1,3 @@
 covalent==0.227.0rc0
 covalent-hpc-plugin==0.0.8
-phonopy==2.20.0
+phonopy==2.21.0

tests/requirements-defects-phonons.txt

@@ -1,3 +1,3 @@
-phonopy==2.20.0
+phonopy==2.21.0
 pymatgen-analysis-defects==2023.10.19
 shakenbreak==3.2.3

tests/requirements-parsl.txt

@@ -1,2 +1,2 @@
 parsl[monitoring]==2023.11.20
-phonopy==2.20.0
+phonopy==2.21.0

tests/requirements-tblite-dftbplus.txt

@@ -1,2 +1,2 @@
-phonopy==2.20.0
+phonopy==2.21.0
 tblite[ase]==0.3.0