diff --git a/breathe/directives/setup.py b/breathe/directives/setup.py
index 58768f89..af0d9b75 100644
--- a/breathe/directives/setup.py
+++ b/breathe/directives/setup.py
@@ -73,22 +73,22 @@ def set_temp_data(
     for name, directive in directives.items():
         app.add_directive(name, directive)
 
-    app.add_config_value("breathe_projects", {}, True)  # Dict[str, str]
-    app.add_config_value("breathe_default_project", "", True)  # str
+    app.add_config_value("breathe_projects", {}, "env")  # Dict[str, str]
+    app.add_config_value("breathe_default_project", "", "env")  # str
     # Provide reasonable defaults for domain_by_extension mapping. Can be overridden by users.
     app.add_config_value(
         "breathe_domain_by_extension", {"py": "py", "cs": "cs"}, True
     )  # Dict[str, str]
-    app.add_config_value("breathe_domain_by_file_pattern", {}, True)  # Dict[str, str]
-    app.add_config_value("breathe_projects_source", {}, True)
-    app.add_config_value("breathe_build_directory", "", True)
-    app.add_config_value("breathe_default_members", (), True)
+    app.add_config_value("breathe_domain_by_file_pattern", {}, "env")  # Dict[str, str]
+    app.add_config_value("breathe_projects_source", {}, "env")
+    app.add_config_value("breathe_build_directory", "", "env")
+    app.add_config_value("breathe_default_members", (), "env")
     app.add_config_value("breathe_show_define_initializer", False, "env")
     app.add_config_value("breathe_show_enumvalue_initializer", False, "env")
     app.add_config_value("breathe_show_include", True, "env")
-    app.add_config_value("breathe_implementation_filename_extensions", [".c", ".cc", ".cpp"], True)
-    app.add_config_value("breathe_doxygen_config_options", {}, True)
-    app.add_config_value("breathe_doxygen_aliases", {}, True)
+    app.add_config_value("breathe_implementation_filename_extensions", [".c", ".cc", ".cpp"], "env")
+    app.add_config_value("breathe_doxygen_config_options", {}, "env")
+    app.add_config_value("breathe_doxygen_aliases", {}, "env")
     app.add_config_value("breathe_use_project_refids", False, "env")
     app.add_config_value("breathe_order_parameters_first", False, "env")
     app.add_config_value("breathe_separate_member_pages", False, "env")
diff --git a/breathe/renderer/filter.py b/breathe/renderer/filter.py
index 31f90ce3..3860b2f9 100644
--- a/breathe/renderer/filter.py
+++ b/breathe/renderer/filter.py
@@ -326,7 +326,7 @@ def __call__(self, node_stack) -> str:
         except AttributeError as e:
             # Horrible hack to silence errors on filtering unicode objects
             # until we fix the parsing
-            if type(data_object) == str:
+            if type(data_object) is str:
                 return "unicode"
             else:
                 raise e
diff --git a/breathe/renderer/sphinxrenderer.py b/breathe/renderer/sphinxrenderer.py
index c2e4ffe9..ea0ecb5f 100644
--- a/breathe/renderer/sphinxrenderer.py
+++ b/breathe/renderer/sphinxrenderer.py
@@ -1,5 +1,4 @@
 import os
-import sphinx
 
 from breathe.parser import compound, compoundsuper, DoxygenCompoundParser
 from breathe.project import ProjectInfo
diff --git a/pyproject.toml b/pyproject.toml
index f3a233a5..df10b389 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -59,6 +59,7 @@ lint = [
     "mypy>=1",
     "types-docutils",
     "types-Pygments",
+    "pytest>=8.0",  # for mypy
 ]
 test = [
     "pytest>=8.0",