bpo-34055: Revert deletion of line in IDLE's PyShell (#11346)

The attribute is still used in other modules.
diff --git a/Lib/idlelib/pyshell.py b/Lib/idlelib/pyshell.py
index 81a97ef..b6172fd 100755
--- a/Lib/idlelib/pyshell.py
+++ b/Lib/idlelib/pyshell.py
@@ -882,7 +882,7 @@
         self.usetabs = True
         # indentwidth must be 8 when using tabs.  See note in EditorWindow:
         self.indentwidth = 8
-
+        self.context_use_ps1 = True
         self.sys_ps1 = sys.ps1 if hasattr(sys, 'ps1') else '>>> '
         self.prompt_last_line = self.sys_ps1.split('\n')[-1]
         self.prompt = self.sys_ps1  # Changes when debug active