Set is_pyi if stdin_filename ends with .pyi

Fixes #2167

src/black/__init__.py

@@ -755,6 +755,8 @@ def reformat_one(
             is_stdin = False
 
         if is_stdin:
+            if src.suffix == ".pyi":
+                mode = replace(mode, is_pyi=True)
             if format_stdin_to_stdout(fast=fast, write_back=write_back, mode=mode):
                 changed = Changed.YES
         else:

tests/test_black.py

@@ -1578,7 +1578,33 @@ def test_reformat_one_with_stdin_filename(self) -> None:
                 mode=DEFAULT_MODE,
                 report=report,
             )
-            fsts.assert_called_once()
+            fsts.assert_called_once_with(
+                fast=True, write_back=black.WriteBack.YES, mode=DEFAULT_MODE
+            )
+            # __BLACK_STDIN_FILENAME__ should have been striped
+            report.done.assert_called_with(expected, black.Changed.YES)
+
+    def test_reformat_one_with_stdin_filename_pyi(self) -> None:
+        with patch(
+            "black.format_stdin_to_stdout",
+            return_value=lambda *args, **kwargs: black.Changed.YES,
+        ) as fsts:
+            report = MagicMock()
+            p = "foo.pyi"
+            path = Path(f"__BLACK_STDIN_FILENAME__{p}")
+            expected = Path(p)
+            black.reformat_one(
+                path,
+                fast=True,
+                write_back=black.WriteBack.YES,
+                mode=DEFAULT_MODE,
+                report=report,
+            )
+            fsts.assert_called_once_with(
+                fast=True,
+                write_back=black.WriteBack.YES,
+                mode=replace(DEFAULT_MODE, is_pyi=True),
+            )
             # __BLACK_STDIN_FILENAME__ should have been striped
             report.done.assert_called_with(expected, black.Changed.YES)