doctest failure when doc html not built/installed but sphinx is available in PYTHONPATH

[tornaria]

Tested on 9.6.beta6:

sage -t --warn-long 59.1 --random-seed=105547399700723726081010659407775307910 src/sage/misc/sagedoc.py
**********************************************************************
File "src/sage/misc/sagedoc.py", line 1404, in sage.misc.sagedoc._sage_doc.__call__
Failed example:
    browse_sage_doc(identity_matrix, 'html', False)             # optional - sphinx
Exception raised:
    Traceback (most recent call last):
      File "/opt/sage/sage-git/develop/pkgs/sagemath-standard/build/lib.linux-x86_64-3.10/sage/doctest/forker.py", line 695, in _run
        self.compile_and_execute(example, compiler, test.globs)
      File "/opt/sage/sage-git/develop/pkgs/sagemath-standard/build/lib.linux-x86_64-3.10/sage/doctest/forker.py", line 1093, in compile_and_execute
        exec(compiled, globs)
      File "<doctest sage.misc.sagedoc._sage_doc.__call__[2]>", line 1, in <module>
        browse_sage_doc(identity_matrix, 'html', False)             # optional - sphinx
      File "sage/misc/lazy_import.pyx", line 391, in sage.misc.lazy_import.LazyImport.__call__ (build/cythonized/sage/misc/lazy_import.c:4256)
        return self.get_object()(*args, **kwds)
      File "/opt/sage/sage-git/develop/pkgs/sagemath-standard/build/lib.linux-x86_64-3.10/sage/misc/sagedoc.py", line 1450, in __call__
        html = sphinxify(s)
      File "/opt/sage/sage-git/develop/pkgs/sagemath-standard/build/lib.linux-x86_64-3.10/sage/misc/sphinxify.py", line 122, in sphinxify
        sphinx_app = Sphinx(srcdir, confdir, outdir, doctreedir, format,
      File "/usr/lib/python3.10/site-packages/sphinx/application.py", line 263, in __init__
        self._init_builder()
      File "/usr/lib/python3.10/site-packages/sphinx/application.py", line 321, in _init_builder
        self.builder.init()
      File "/usr/lib/python3.10/site-packages/sphinx/builders/html/__init__.py", line 220, in init
        self.init_templates()
      File "/usr/lib/python3.10/site-packages/sphinx/builders/html/__init__.py", line 268, in init_templates
        self.theme = theme_factory.create(themename)
      File "/usr/lib/python3.10/site-packages/sphinx/theming.py", line 249, in create
        raise ThemeError(__('no theme named %r found (missing theme.conf?)') % name)
    sphinx.errors.ThemeError: no theme named 'sage-classic' found (missing theme.conf?)
**********************************************************************
1 item had failures:
   1 of   6 in sage.misc.sagedoc._sage_doc.__call__
    [100 tests, 1 failure, 20.72 s]

Indeed, this is not new, but I didn't have sphinx installed before.

It seems to me that the following patch should fix this:

--- a/src/sage/misc/sagedoc.py
+++ b/src/sage/misc/sagedoc.py
@@ -1401,7 +1401,7 @@ class _sage_doc:
             "...**File:**...**Type:**...**Definition:** identity_matrix..."
             sage: identity_matrix.__doc__ in browse_sage_doc(identity_matrix, 'rst')
             True
-            sage: browse_sage_doc(identity_matrix, 'html', False)             # optional - sphinx
+            sage: browse_sage_doc(identity_matrix, 'html', False)             # optional - sphinx sagemath_doc_html
             '...div...File:...Type:...Definition:...identity_matrix...'
 
         In the 'text' version, double colons have been replaced with

Component: doctest framework

Author: Gonzalo Tornaría

Branch/Commit: a04a0a0

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/33585

[tornaria]

New commits:

a04a0a0

Trac #33585: fix doctest when dochtml is missing...

[tornaria]

Author: Gonzalo Tornaría

[tornaria]

Commit: a04a0a0

[tornaria]

Branch: u/tornaria/33585

[mkoeppe]

comment:2

LGTM.

[mkoeppe]

Reviewer: Matthias Koeppe

[vbraun]

Changed branch from u/tornaria/33585 to a04a0a0