Avoid RemovedInSphinx80Warning in path-manipulation code.

In recent versions of Sphinx, code that treats Sphinx application
paths such as `Sphinx.confdir` and `Sphinx.doctreedir` as strings
emits the warning "Sphinx 8 will drop support for representing paths
as strings".

This commit updates path-manipulation code to use functions in the
`os.path` module, which accept path-like objects as well as strings.

This fixes issue breathe-doc#974.

breathe/project.py

@@ -110,10 +110,9 @@ def __init__(self, app: Sphinx):
         self.app = app
         # note: don't access self.app.config now, as we are instantiated at setup-time.
 
-        # Assume general build directory is the doctree directory without the last component.
-        # We strip off any trailing slashes so that dirname correctly drops the last part.
+        # Assume general build directory is the parent of the doctree directory.
         # This can be overridden with the breathe_build_directory config variable
-        self._default_build_dir = os.path.dirname(app.doctreedir.rstrip(os.sep))
+        self._default_build_dir = os.path.dirname(os.path.normpath(app.doctreedir))
         self.project_count = 0
         self.project_info_store: Dict[str, ProjectInfo] = {}
         self.project_info_for_auto_store: Dict[str, AutoProjectInfo] = {}

breathe/renderer/sphinxrenderer.py

@@ -2411,12 +2411,9 @@ def visit_docdotfile(self, node) -> List[Node]:
             # Use self.project_info.project_path as the XML_OUTPUT path, and
             # make it absolute with consideration to the conf.py path
             project_path = self.project_info.project_path()
-            if os.path.isabs(project_path):
-                dot_file_path = os.path.abspath(project_path + os.sep + dot_file_path)
-            else:
-                dot_file_path = os.path.abspath(
-                    self.app.confdir + os.sep + project_path + os.sep + dot_file_path
-                )
+            dot_file_path = os.path.abspath(
+                os.path.join(self.app.confdir, project_path, dot_file_path)
+            )
         try:
             with open(dot_file_path, encoding="utf-8") as fp:
                 dotcode = fp.read()