[REM] pycompat: remove python 2 shims and helpers

Odoo no longer supports python 2, thus some of these helpers can and
have been replaced by python 3 built-ins, therefore there is no need for
them to stay defined.

The removed helpers are:
    * izip, imap and ifilter
    * unichr, text_type
    * implements_to_string, implements_iterator
    * string_types, integer_types
    * to_native

The python 2 shims have also been removed, and only the python 3 helpers
have been kept, because they can still be usable (i.e. accepting
both bytes and str for functions that can only accept one of the two)

[REM] pyjsparser: remove PY3 shims

They're no longer necessary as Odoo doesn't officially support python 2
anymore.

closes odoo/odoo#28519

doc/python3.rst

@@ -62,16 +62,6 @@ features whereas:
 * Python 3.2 made ``range`` views more list-like (backported to 2.7)and
   reintroduced ``callable``
 
-.. warning::
-
-    While Python 3 adds plenty of great features (keyword-only parameters,
-    generator delegation, pathlib, ...), you must *not* use them in Odoo
-    until Python 2 support is dropped
-
-.. note::
-
-    In the *very rare* cases where you *need* to differentiate between
-    Python 2 and Python 3, use the :data:`odoo.tools.pycompat.PY2` flag.
 
 Semantics changes
 =================