code: make TracebackEntry immutable

TracebackEntry being mutable caught me by surprise and makes reasoning
about the exception formatting code harder. Make it a proper value.

src/_pytest/_code/code.py

@@ -49,6 +49,7 @@ from _pytest.pathlib import absolutepath
 from _pytest.pathlib import bestrelpath
 
 if TYPE_CHECKING:
+    from typing_extensions import Final
     from typing_extensions import Literal
     from typing_extensions import SupportsIndex
 
@@ -197,18 +198,20 @@ class TracebackEntry:
     def __init__(
         self,
         rawentry: TracebackType,
+        repr_style: Optional['Literal["short", "long"]'] = None,
     ) -> None:
-        self._rawentry = rawentry
-        self._repr_style: Optional['Literal["short", "long"]'] = None
+        self._rawentry: "Final" = rawentry
+        self._repr_style: "Final" = repr_style
+
+    def with_repr_style(
+        self, repr_style: Optional['Literal["short", "long"]']
+    ) -> "TracebackEntry":
+        return TracebackEntry(self._rawentry, repr_style)
 
     @property
     def lineno(self) -> int:
         return self._rawentry.tb_lineno - 1
 
-    def set_repr_style(self, mode: "Literal['short', 'long']") -> None:
-        assert mode in ("short", "long")
-        self._repr_style = mode
-
     @property
     def frame(self) -> Frame:
         return Frame(self._rawentry.tb_frame)

src/_pytest/python.py

@@ -35,6 +35,7 @@ from _pytest._code import filter_traceback
 from _pytest._code import getfslineno
 from _pytest._code.code import ExceptionInfo
 from _pytest._code.code import TerminalRepr
+from _pytest._code.code import Traceback
 from _pytest._io import TerminalWriter
 from _pytest._io.saferepr import saferepr
 from _pytest.compat import ascii_escaped
@@ -1819,8 +1820,12 @@ class Function(PyobjMixin, nodes.Item):
             # only show a single-line message for each frame.
             if self.config.getoption("tbstyle", "auto") == "auto":
                 if len(excinfo.traceback) > 2:
-                    for entry in excinfo.traceback[1:-1]:
-                        entry.set_repr_style("short")
+                    excinfo.traceback = Traceback(
+                        entry
+                        if i == 0 or i == len(excinfo.traceback) - 1
+                        else entry.with_repr_style("short")
+                        for i, entry in enumerate(excinfo.traceback)
+                    )
 
     # TODO: Type ignored -- breaks Liskov Substitution.
     def repr_failure(  # type: ignore[override]

testing/code/test_excinfo.py

@@ -1122,8 +1122,10 @@ raise ValueError()
         )
         excinfo = pytest.raises(ValueError, mod.f)
         excinfo.traceback = excinfo.traceback.filter(excinfo)
-        excinfo.traceback[1].set_repr_style("short")
-        excinfo.traceback[2].set_repr_style("short")
+        excinfo.traceback = _pytest._code.Traceback(
+            entry if i not in (1, 2) else entry.with_repr_style("short")
+            for i, entry in enumerate(excinfo.traceback)
+        )
         r = excinfo.getrepr(style="long")
         r.toterminal(tw_mock)
         for line in tw_mock.lines: