erp5: Switch from python-interpreter to gpython-interpreter

This is preparatory step to later use bstr/ustr from gpython as builtin string
types. For now we keep both runtime to be threads and string types to be
unpatched builtin types provided by python natively. This way there should be
practically no change in how ERP5 runs yet.

/cc @kazuhiko, @jerome

component/pygolang/buildout.cfg

@@ -41,7 +41,9 @@ eggs   = ${pygolang:egg}
 interpreter = python
 # interpreter code that buildout generates cannot process `-m pytest --<pytest-option>`
 # -> use pymain from gpython to workaround that.
+pre-initialization =
 initialization =
+  ${:pre-initialization}
   # tail to pymain
   from gpython import pymain
   pymain(sys.argv)
@@ -60,6 +62,7 @@ exe     = ${buildout:bin-directory}/${:interpreter}
 <= python-interpreter
 interpreter = gpython
 initialization =
+  ${:pre-initialization}
   from gpython import main
   main()
   sys.exit(0)

stack/erp5/buildout.cfg

@@ -463,9 +463,16 @@ initialization =
   sys.path[0:0] = ['/'.join(['''${buildout:parts-directory}''', x]) for x in repository_id_list]
 
 [erp5-python-interpreter]
-<= python-interpreter
 # a python interpreter with all eggs available, usable for the software release but also
 # for external tools (such as python extension in theia).
+# we will also want to use bstr/ustr from gpython as builtin string types, but keep
+# the program named as just `python`.
+<= gpython-interpreter
+interpreter = python
+pre-initialization +=
+  import os
+  os.environ.setdefault('GPYTHON_STRINGS', 'pystd')    # TODO switch to bstr+ustr
+  os.environ.setdefault('GPYTHON_RUNTIME', 'threads')
 eggs += ${eggs:eggs}
 extra-paths += ${eggs:extra-paths}