terminalwriter: remove TerminalWriter's stringio argument

Had a mark indicating it should be removed, and I agree, it's better to
just use the `file` argument.

src/_pytest/_io/terminalwriter.py

@@ -4,7 +4,6 @@ import shutil
 import sys
 import unicodedata
 from functools import lru_cache
-from io import StringIO
 from typing import Sequence
 
 
@@ -83,13 +82,9 @@ class TerminalWriter:
         invert=7,
     )
 
-    # XXX deprecate stringio argument
-    def __init__(self, file=None, stringio=False):
+    def __init__(self, file=None):
         if file is None:
-            if stringio:
-                self.stringio = file = StringIO()
-            else:
-                from sys import stdout as file
+            file = sys.stdout
         if hasattr(file, "isatty") and file.isatty() and colorama:
             file = colorama.AnsiToWin32(file).stream
         self._file = file

src/_pytest/pastebin.py

@@ -1,5 +1,6 @@
 """ submit failure or test session information to a pastebin service. """
 import tempfile
+from io import StringIO
 from typing import IO
 
 import pytest
@@ -99,11 +100,10 @@ def pytest_terminal_summary(terminalreporter):
                 msg = rep.longrepr.reprtraceback.reprentries[-1].reprfileloc
             except AttributeError:
                 msg = tr._getfailureheadline(rep)
-            tw = _pytest.config.create_terminal_writer(
-                terminalreporter.config, stringio=True
-            )
+            file = StringIO()
+            tw = _pytest.config.create_terminal_writer(terminalreporter.config, file)
             rep.toterminal(tw)
-            s = tw.stringio.getvalue()
+            s = file.getvalue()
             assert len(s)
             pastebinurl = create_new_paste(s)
             tr.write_line("{} --> {}".format(msg, pastebinurl))

src/_pytest/reports.py

@@ -82,10 +82,11 @@ class BaseReport:
 
         .. versionadded:: 3.0
         """
-        tw = TerminalWriter(stringio=True)
+        file = StringIO()
+        tw = TerminalWriter(file)
         tw.hasmarkup = False
         self.toterminal(tw)
-        exc = tw.stringio.getvalue()
+        exc = file.getvalue()
         return exc.strip()
 
     @property