Fix pdf docbuild failure if destination file exists

[vbraun]

The destination of shutil.move must be the filename and not a directory to take advantage of move-overwrite filesystem semantic.

Fixes #40854

[github-actions]

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[user202729]

looking at the source code of shutil.move, the explanation is actually not true (os.rename is performed either way, just that an additional check is performed when the target is a directory), but as far as I can tell the net effect is the same.

def move(src, dst, copy_function=copy2):
    sys.audit("shutil.move", src, dst)
    real_dst = dst
    if os.path.isdir(dst):
		...
        # Using _basename instead of os.path.basename is important, as we must
        # ignore any trailing slash to avoid the basename returning ''
        real_dst = os.path.join(dst, _basename(src))

        if os.path.exists(real_dst):  # <<< the check here
            raise Error("Destination path '%s' already exists" % real_dst)
    try:
        os.rename(src, real_dst)
    except OSError:
		...
    return real_dst

[vbraun]

I stand corrected...