Documentation does not build with Sphinx 4.1.2

lib/spack/docs/requirements.txt

@@ -1,7 +1,7 @@
 # These dependencies should be installed using pip in order
 # to build the documentation.
 
-sphinx>=3.4
+sphinx>=3.4,!=4.1.2
 sphinxcontrib-programoutput
 sphinx-rtd-theme
 python-levenshtein

lib/spack/docs/spack.yaml

@@ -8,12 +8,20 @@
 # these commands in this directory to install Sphinx and its plugins,
 # then build the docs:
 #
-#     spack install
 #     spack env activate .
+#     spack install
 #     make
 #
 spack:
   specs:
-  - "py-sphinx@3.4:"
+  # Sphinx
+  - "py-sphinx@3.4:4.1.1,4.1.3:"
   - py-sphinxcontrib-programoutput
   - py-sphinx-rtd-theme
+  # VCS
+  - git
+  - mercurial
+  - subversion
+  # Plotting
+  - graphviz
+  concretization: together

lib/spack/spack/package.py

@@ -2657,7 +2657,7 @@ def test_process(pkg, kwargs):
 
 
 class BundlePackage(PackageBase):
-    #"""General purpose bundle, or no-code, package class."""
+    """General purpose bundle, or no-code, package class."""
     #: There are no phases by default but the property is required to support
     #: post-install hooks (e.g., for module generation).
     phases = []  # type: List[str]