Use TOMLFile.write for saving and remove newline

src/poetry/console/commands/self/self_command.py

@@ -77,10 +77,8 @@ def generate_system_pyproject(self) -> None:
         for key in preserved:
             content["tool"]["poetry"][key] = preserved[key]  # type: ignore[index]
 
-        text = content.as_string().replace("\r\n", "\n")  # remove carriage return
-        if text.endswith("\n\n"):
-            text = text[:-1]  # remove extra newline if trailing newline already present
-        self.system_pyproject.write_text(text, encoding="utf-8")
+        pyproject = PyProjectTOML(self.system_pyproject)
+        pyproject.file.write(content)
 
     def reset_poetry(self) -> None:
         with directory(self.system_pyproject.parent):

src/poetry/factory.py

@@ -290,7 +290,6 @@ def create_pyproject_from_package(
             content["extras"] = extras_section
 
         pyproject = cast("TOMLDocument", pyproject)
-        pyproject.add(tomlkit.nl())
 
         if path:
             path.joinpath("pyproject.toml").write_text(

tests/console/commands/self/test_system_pyproject.py

@@ -20,7 +20,7 @@ def example_system_pyproject():
         Dependency(plugin.name, "^1.2.3", groups=[SelfCommand.ADDITIONAL_PACKAGE_GROUP])
     )
     content = Factory.create_pyproject_from_package(package)
-    return content.as_string()[:-2]  # remove trailing newlines
+    return content.as_string().strip("\n")
 
 
 @pytest.mark.parametrize("existing_newlines", [0, 2])
@@ -37,9 +37,7 @@ def test_generate_system_pyproject_trailing_newline(
         if c != "\n":
             break
 
-    assert (existing_newlines < 2 and _i == 1) or (
-        existing_newlines > 1 and _i == existing_newlines
-    )
+    assert _i == existing_newlines
 
 
 def test_generate_system_pyproject_carraige_returns(