CI: cleanup upper limits as these should be picked up by the devdeps job

Author: bsipocz

.github/workflows/tests.yml

@@ -20,8 +20,8 @@ jobs:
       matrix:
         os: [ubuntu-latest]
         python-version: ["3.9", "3.10", "3.11", "3.12"]
-        sphinx: [">=5,<9"]  # Newest Sphinx (any)
-        myst-parser: [">=1,<3"]  # Newest MyST Parser (any)
+        sphinx: [""]  # Newest Sphinx (any)
+        myst-parser: [""]  # Newest MyST Parser (any)
         include:
           # Just check the other platforms once
           - os: windows-latest