Fix out of scope error when "dest" variable is undefined

Fixes theskumar#413 whereby the NamedTemporaryFile "dest" was out of scope in the
error handling portion of rewrite.

The problem was initially fixed in theskumar#414 but it got reverted because of a
linter error. This new commit works around that linter error.

src/dotenv/main.py

@@ -125,19 +125,17 @@ def rewrite(
     path: Union[str, os.PathLike],
     encoding: Optional[str],
 ) -> Iterator[Tuple[IO[str], IO[str]]]:
-    try:
-        if not os.path.isfile(path):
-            with open(path, "w+", encoding=encoding) as source:
-                source.write("")
-        with tempfile.NamedTemporaryFile(mode="w+", delete=False, encoding=encoding) as dest:
+    if not os.path.isfile(path):
+        with open(path, mode="w", encoding=encoding) as source:
+            source.write("")
+    with tempfile.NamedTemporaryFile(mode="w", encoding=encoding, delete=False) as dest:
+        try:
             with open(path, encoding=encoding) as source:
-                yield (source, dest)  # type: ignore
-    except BaseException:
-        if os.path.isfile(dest.name):
+                yield (source, dest)
+        except BaseException:
             os.unlink(dest.name)
-        raise
-    else:
-        shutil.move(dest.name, path)
+            raise
+    shutil.move(dest.name, path)
 
 
 def set_key(

tests/test_main.py

@@ -178,6 +178,13 @@ def test_unset_encoding(dotenv_file):
         assert f.read() == ""
 
 
+def test_set_key_unauthorized_file(dotenv_file):
+    os.chmod(dotenv_file, 0o000)
+
+    with pytest.raises(PermissionError):
+        dotenv.set_key(dotenv_file, "a", "x")
+
+
 def test_unset_non_existent_file(tmp_path):
     nx_file = str(tmp_path / "nx")
     logger = logging.getLogger("dotenv.main")