Fix dynamic readme

Author: abravalheri

setuptools/config/pyprojecttoml.py

@@ -316,12 +316,17 @@ def _obtain_version(self, dist: "Distribution", package_dir: Mapping[str, str]):
         return None
 
     def _obtain_readme(self, dist: "Distribution") -> Optional[Dict[str, str]]:
-        if "readme" in self.dynamic:
-            dynamic_cfg = self.dynamic_cfg
+        if "readme" not in self.dynamic:
+            return None
+
+        dynamic_cfg = self.dynamic_cfg
+        if "readme" in dynamic_cfg:
             return {
                 "text": self._obtain(dist, "readme", {}),
                 "content-type": dynamic_cfg["readme"].get("content-type", "text/x-rst"),
             }
+
+        self._ensure_previously_set(dist, "readme")
         return None
 
     def _obtain_entry_points(