gpython: Implement -E

Let's teach gpython and pymain about -E (ignore $PYTHON* environment
variables) because new buildout runs python -E inside. Xavier reports:

    Since slapos was upgraded zc.buildout 3.0.1+slapos004, tests for
    slapos.rebootstrap and slapos.recipe.template fail because buildout now
    installs in develop with pip install --editable instead of python
    setup.py develop and in the process pip runs python -E, e.g.
    https://erp5js.nexedi.net/#/test_result_module/20240912-837A12F7/10

For the implementation use the same approach to reexecute underlying
interpreter with given low-level option as we already did for -O in
8564dfdd (gpython: Implement -O).

/reported-by @xavier_thompson
/cc @jerome

gpython/__init__.py

 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
-# Copyright (C) 2018-2023  Nexedi SA and Contributors.
+# Copyright (C) 2018-2024  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -41,7 +41,7 @@ $GPYTHON_RUNTIME=threads.
 from __future__ import print_function, absolute_import
 
 
-_pyopt = "c:im:OVW:X:"
+_pyopt = "c:Eim:OVW:X:"
 _pyopt_long = ('version',)
 
 # pymain mimics `python ...`
@@ -113,6 +113,7 @@ def pymain(argv, init=None):
     for (opt, arg) in igetopt:
         # options that require reexecuting through underlying python with that -<opt>
         if opt in (
+                '-E',   # ignore $PYTHON*
                 '-O',   # optimize
             ):
             reexec_with.append(opt)

gpython/gpython_test.py

@@ -315,6 +315,28 @@ def test_pymain_opt():
         check(["-O", "-O"])
         check(["-O", "-O", "-O"])
 
+# verify that pymain handles -E in exactly the same way as underlying python does.
+@gpython_only
+def test_pymain_E():
+    envadj = {'PYTHONOPTIMIZE': '1'}
+    def sys_flags_optimize(level):
+        return 'sys.flags.optimize:   %s' % level
+
+    # without -E $PYTHONOPTIMIZE should be taken into account
+    def _(gpyoutv, stdpyoutv):
+        assert sys_flags_optimize(0) not in stdpyoutv
+        assert sys_flags_optimize(0) not in gpyoutv
+        assert sys_flags_optimize(1)     in stdpyoutv
+        assert sys_flags_optimize(1)     in gpyoutv
+    check_gpy_vs_py(['testprog/print_opt.py'], _, envadj=envadj, cwd=here)
+
+    # with -E not
+    def _(gpyoutv, stdpyoutv):
+        assert sys_flags_optimize(0)     in stdpyoutv
+        assert sys_flags_optimize(0)     in gpyoutv
+        assert sys_flags_optimize(1) not in stdpyoutv
+        assert sys_flags_optimize(1) not in gpyoutv
+    check_gpy_vs_py(['-E', 'testprog/print_opt.py'], _, envadj=envadj, cwd=here)
 
 # pymain -V/--version
 # gpython_only because output differs from !gpython.