[IMP] base: "--shell-file" option override for $PYTHONSTARTUP

The $PYTHONSTARTUP env variable can contain a python script
that is used by Python shell at the start of any interactive session.

- fix this feature from being broken in python and ptpython
- include a new --shell-file=<filename> shell parameter to override
  the env variable $PYTHONSTARTUP
- group the two shell parameters in a new options group
- remove custom python shells' banners from the start of the session
- shell options can now be saved in the configuration file.

Python docs: https://docs.python.org/3/using/cmdline.html#envvar-PYTHONSTARTUP

Community PR: odoo/odoo#185075
task-4306704

Author: lordkrandel

content/developer/reference/cli.rst

@@ -695,6 +695,11 @@ interaction with the :ref:`orm <reference/orm>` and its functionalities.
       By default, the shell is running in transaction mode. This means that any change made to the
       database is rolled back when exiting the shell. To commit changes, use `env.cr.commit()`.
 
+
+.. option:: --shell-file <init_script.py>
+   Specify a python script to be run after the start of the shell.
+   Overrides the environment variable PYTHONSTARTUP.
+
 .. option:: --shell-interface (ipython|ptpython|bpython|python)
 
    Specify a preferred REPL to use in shell mode. This shell is started with the `env` variable