gh-46927: Prevent readline from overriding environment

Readline library will set the LINES and COLUMNS environment variables
during initialization. One might expect these variables to be updated
later on SIGWINCH, but this is not the case with cpython.

As a consequence, when the readline module is imported, any process
launched from cpython with the default environment will have LINES and
COLUMNS variables set, potentially to a wrong value.

Use the rl_change_environment global variable to disable this initial
setup of the environment variables.

Author: mbriand

Misc/NEWS.d/next/Core_and_Builtins/2025-05-07-18-31-31.gh-issue-46927.sF02gj.rst

@@ -0,0 +1,2 @@
+Prevent readline from overriding COLUMNS and LINES environment variables, as
+values are not updated on terminal resize.

Modules/readline.c

@@ -1323,6 +1323,13 @@ setup_readline(readlinestate *mod_state)
     /* The name must be defined before initialization */
     rl_readline_name = "python";
 
+#ifndef WITH_EDITLINE
+    /* Prevent readline from changing environment variables such as LINES and
+     * COLUMNS.
+     */
+    rl_change_environment = 0;
+#endif
+
     /* the libedit readline emulation resets key bindings etc
      * when calling rl_initialize.  So call it upfront
      */