Uncap the Sphinx requirement (#2952)

This updates the Sphinx requirement to allow 6.1.2 and beyond, but blacklists 6.1.0 and 6.1.1 (broken versions) from being installed.

Author: AA-Turner

requirements.txt

@@ -1,6 +1,8 @@
 # Requirements for building PEPs with Sphinx
 Pygments >= 2.9.0
-Sphinx >= 5.1.1,<=6.0.0
+# Sphinx 6.1.0 broke copying images in parallel builds; fixed in 6.1.2
+# See https://github.com/sphinx-doc/sphinx/pull/11100
+Sphinx >= 5.1.1, != 6.1.0, != 6.1.1
 docutils >= 0.19.0
 
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