Update tests.yaml

Author: kujaku11

.github/workflows/tests.yaml

@@ -36,7 +36,7 @@ jobs:
         uv venv --python ${{ matrix.python-version }}
         uv pip install -e ".[dev,test]"
         uv pip install "mt_metadata[obspy] @ git+https://github.com/kujaku11/mt_metadata.git@pydantic"
-        uv pip install git+https://github.com/kujaku11/mth5.git@pydantic
+        uv pip install git+https://github.com/kujaku11/mth5.git@old_pydantic
         uv pip install jupyter ipykernel pytest pytest-cov codecov
 
     - name: Install system dependencies