Remove version upper bound on pyyaml (#2514)

* Remove version upper bound on `pyyaml`

* Disallow `5.4.*` due to incompatibility with cython3

Co-authored-by: Romil Bhardwaj <romil.bhardwaj@gmail.com>

* Update comment for PyYAML pinned versions

---------

Co-authored-by: Romil Bhardwaj <romil.bhardwaj@gmail.com>

sky/setup_files/setup.py

@@ -109,11 +109,9 @@ def parse_readme(readme: str) -> str:
     # >=1.10.8 is needed for ray>=2.6. See
     # https://github.com/ray-project/ray/issues/35661
     'pydantic <2.0, >=1.10.8',
-    # Cython 3.0 release breaks PyYAML installed by aws-cli.
-    # https://github.com/yaml/pyyaml/issues/601
-    # https://github.com/aws/aws-cli/issues/8036
+    # Cython 3.0 release breaks PyYAML 5.4.* (https://github.com/yaml/pyyaml/issues/601)
     # <= 3.13 may encounter https://github.com/ultralytics/yolov5/issues/414
-    'pyyaml > 3.13, <= 5.3.1'
+    'pyyaml > 3.13, != 5.4.*'
 ]
 
 # NOTE: Change the templates/spot-controller.yaml.j2 file if any of the