fix: Disallow Sphinx v5.1.0 to avoid 'exception: pop from an empty de…

…que' (#1925)

* Disallow Sphinx v5.1.0 as it introduces an deque error that can be
  caused by sphinx.ext.napoleon:
> Extension error (sphinx.ext.napoleon):
> Handler <function _process_docstring at 0x7f43e8361a20> for event 'autodoc-process-docstring'
> threw an exception (exception: pop from an empty deque)
   - This was fixed rapidly in sphinx-doc/sphinx#10709 and
     so will not be a problem in Sphinx v5.1.1+.

setup.py

@@ -50,7 +50,7 @@
         extras_require['xmlio']
         + extras_require['contrib']
         + [
-            'sphinx>=4.0.0',
+            'sphinx>=4.0.0,!=5.1.0',  # c.f. https://github.com/scikit-hep/pyhf/pull/1925
             'sphinxcontrib-bibtex~=2.1',
             'sphinx-click',
             'sphinx_rtd_theme',