build: remove sphinx version pin (#41)

Required version of sphinx is already pinned by sphinx-rtd-theme.
They should update their version range when they add sphinx v8
support.

setup.py

@@ -24,7 +24,7 @@ def main():
                           'tables'],
         extras_require={
             'test': ['pytest', 'nbval', 'django', 'nbformat'],
-            'docs': ['sphinx<8', 'sphinx-rtd-theme', 'mock', 'nbsphinx',
+            'docs': ['sphinx', 'sphinx-rtd-theme', 'mock', 'nbsphinx',
                      'ipykernel'],
         },
         cmdclass=versioneer.get_cmdclass(),