diff --git a/mypy.ini b/mypy.ini
index c5b13942c4..a63ccf787c 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -4,6 +4,7 @@
 python_version = 3.8
 strict = False
 warn_unused_ignores = True
+warn_redundant_casts = True
 # required to support namespace packages: https://github.com/python/mypy/issues/14057
 explicit_package_bases = True
 exclude = (?x)(
diff --git a/setuptools/command/editable_wheel.py b/setuptools/command/editable_wheel.py
index 55d477eebf..ae31bb4c79 100644
--- a/setuptools/command/editable_wheel.py
+++ b/setuptools/command/editable_wheel.py
@@ -515,7 +515,7 @@ def template_vars(self) -> tuple[str, str, dict[str, str], dict[str, list[str]]]
         )
 
         legacy_namespaces = {
-            cast(str, pkg): find_package_path(pkg, roots, self.dist.src_root or "")
+            pkg: find_package_path(pkg, roots, self.dist.src_root or "")
             for pkg in self.dist.namespace_packages or []
         }
 
diff --git a/setuptools/config/_apply_pyprojecttoml.py b/setuptools/config/_apply_pyprojecttoml.py
index c7e25b755f..5a8700051e 100644
--- a/setuptools/config/_apply_pyprojecttoml.py
+++ b/setuptools/config/_apply_pyprojecttoml.py
@@ -12,7 +12,6 @@
 
 import logging
 import os
-from collections.abc import Mapping
 from email.headerregistry import Address
 from functools import partial, reduce
 from inspect import cleandoc
@@ -22,8 +21,9 @@
     TYPE_CHECKING,
     Any,
     Callable,
+    Dict,
+    Mapping,
     Union,
-    cast,
 )
 from .._path import StrPath
 from ..errors import RemovedConfigError
@@ -35,7 +35,7 @@
     from setuptools.dist import Distribution  # noqa
 
 EMPTY: Mapping = MappingProxyType({})  # Immutable dict-like
-_DictOrStr = Union[dict, str]
+_ProjectReadmeValue = Union[str, Dict[str, str]]
 _CorrespFn = Callable[["Distribution", Any, StrPath], None]
 _Correspondence = Union[str, _CorrespFn]
 
@@ -149,15 +149,16 @@ def _guess_content_type(file: str) -> str | None:
     raise ValueError(f"Undefined content type for {file}, {msg}")
 
 
-def _long_description(dist: Distribution, val: _DictOrStr, root_dir: StrPath):
+def _long_description(dist: Distribution, val: _ProjectReadmeValue, root_dir: StrPath):
     from setuptools.config import expand
 
+    file: str | tuple[()]
     if isinstance(val, str):
-        file: str | list = val
+        file = val
         text = expand.read_files(file, root_dir)
-        ctype = _guess_content_type(val)
+        ctype = _guess_content_type(file)
     else:
-        file = val.get("file") or []
+        file = val.get("file") or ()
         text = val.get("text") or expand.read_files(file, root_dir)
         ctype = val["content-type"]
 
@@ -167,7 +168,7 @@ def _long_description(dist: Distribution, val: _DictOrStr, root_dir: StrPath):
         _set_config(dist, "long_description_content_type", ctype)
 
     if file:
-        dist._referenced_files.add(cast(str, file))
+        dist._referenced_files.add(file)
 
 
 def _license(dist: Distribution, val: dict, root_dir: StrPath):
diff --git a/setuptools/config/expand.py b/setuptools/config/expand.py
index 7140dc8ed8..6ea6cf6d0e 100644
--- a/setuptools/config/expand.py
+++ b/setuptools/config/expand.py
@@ -36,8 +36,6 @@
     Iterator,
     Mapping,
     TypeVar,
-    Union,
-    cast,
 )
 from pathlib import Path
 from types import ModuleType
@@ -326,18 +324,13 @@ def version(value: Callable | Iterable[str | int] | str) -> str:
     """When getting the version directly from an attribute,
     it should be normalised to string.
     """
-    if callable(value):
-        value = value()
+    _value = value() if callable(value) else value
 
-    value = cast(Iterable[Union[str, int]], value)
-
-    if not isinstance(value, str):
-        if hasattr(value, '__iter__'):
-            value = '.'.join(map(str, value))
-        else:
-            value = '%s' % value
-
-    return value
+    if isinstance(_value, str):
+        return _value
+    if hasattr(_value, '__iter__'):
+        return '.'.join(map(str, _value))
+    return '%s' % _value
 
 
 def canonic_package_data(package_data: dict) -> dict:
diff --git a/setuptools/dist.py b/setuptools/dist.py
index 30cdfdb10b..43762960ba 100644
--- a/setuptools/dist.py
+++ b/setuptools/dist.py
@@ -272,6 +272,8 @@ class Distribution(_Distribution):
     }
 
     _patched_dist = None
+    # Used by build_py, editable_wheel and install_lib commands for legacy namespaces
+    namespace_packages: list[str]  #: :meta private: DEPRECATED
 
     def patch_missing_pkg_info(self, attrs):
         # Fake up a replacement for the data that would normally come from