[tarfile] Fix pax_headers type annotation in tarfile.pyi (#15215)

Author: guoci

stdlib/tarfile.pyi

@@ -127,7 +127,7 @@ class TarFile:
     encoding: str | None
     errors: str
     fileobject: type[ExFileObject]  # undocumented
-    pax_headers: Mapping[str, str] | None
+    pax_headers: Mapping[str, str]
     debug: int | None
     errorlevel: int | None
     offset: int  # undocumented