[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have been replaced by python 3 built-ins, therefore there is no need for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