Fix empty _sources directory created when html_copy_source is False

[FazeelUsmani]

This PR fixes a regression introduced in v8.2.0 where an empty _sources/ directory is created during HTML builds even when html_copy_source = False is explicitly set in conf.py.

The Bug:
The prepare_writing() method in StandaloneHTMLBuilder was checking self.copysource (a class attribute that always defaults to True) instead of self.config.html_copy_source (the actual user configuration value).

The Fix:
Changed the condition on line 574 of sphinx/builders/html/__init__.py from:

if self.copysource:
to:
if self.config.html_copy_source:

Testing:

• Verified the fix resolves the issue with manual reproduction steps
• All existing tests pass, including test_html_copy_source
• Confirmed both scenarios work correctly:
  • html_copy_source = False: No _sources directory created ✓
  • html_copy_source = True: _sources directory created with source files ✓

This is important for users packaging documentation (e.g., Debian packages) who should not have to manually clean up empty directories that shouldn't exist.

References

• Fixes Empty _sources directory created even with html_copy_source disabled #13888
• Regression introduced in commit 285908a (PR Use _StrPath in HTMLBuilder #13045)