sagemathgh-38608: Add warning filter for DeprecationWarning while doc…

… build

    
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Fixes sagemath#37664.

from the log of the doc build workflow of this PR,
```
  [sagemath_doc_html-none] [spkg-install] sage --docbuild --no-pdf-links
reference/cryptography inventory
  [sagemath_doc_html-none] [spkg-install] [cryptogra] Converting
`source_suffix = '.rst'` to `source_suffix = {'.rst':
'restructuredtext'}`.
  [sagemath_doc_html-none] [spkg-install] [cryptogra] building
[inventory]: targets for 25 source files that are out of date
  [sagemath_doc_html-none] [spkg-install] [cryptogra] updating
environment: [new config] 25 added, 0 changed, 0 removed
  [sagemath_doc_html-none] [spkg-install] [cryptogra] The inventory file
is in ../../local/share/doc/sage/inventory/en/reference/cryptography.
  [sagemath_doc_html-none] [spkg-install] sage --docbuild --no-pdf-links
reference/curves inventory
```

This PR is based on sagemath#38580 and replaces it. Thank @thecaligarmo for
allowing this arrangement.

### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#38608
Reported by: Kwankyu Lee
Reviewer(s): Matthias Köppe

src/sage_docbuild/sphinxbuild.py

@@ -307,6 +307,11 @@ def runsphinx():
     saved_stdout = sys.stdout
     saved_stderr = sys.stderr
 
+    if not sys.warnoptions:
+        import warnings
+        original_filters = warnings.filters[:]
+        warnings.filterwarnings("ignore", category=DeprecationWarning, module='sphinx.util.inspect')
+
     try:
         sys.stdout = SageSphinxLogger(sys.stdout, os.path.basename(output_dir))
         sys.stderr = SageSphinxLogger(sys.stderr, os.path.basename(output_dir))
@@ -323,3 +328,6 @@ def runsphinx():
         sys.stderr = saved_stderr
         sys.stdout.flush()
         sys.stderr.flush()
+
+    if not sys.warnoptions:
+        warnings.filters = original_filters[:]