[REF] *: replace deprecated pycompat helpers for builtins

This commit replaces calls to pycompat helpers that were intended for
python 2 <-> python 3 interoperability for python 3 builtins, as python
2 is no longer officially supported by Odoo.

This includes:
    * calls to imap/izip/ifilter replaced by map/zip/filter
    * uses of text_type replaced by str
    * uses of unichr replaced by chr
    * calls to implements_to_string, implements_iterator removed
    * string_types and integer_types replaced by str, int respectively
    * calls to to_native replaced by calls to to_text

This is done in preparation to the removal of these deprecated helpers
in the following commit.

doc/python3.rst

@@ -299,8 +299,6 @@ under the single ``int`` type.
 
     * the ``L`` suffix for integer literals must be removed
     * calls to ``long`` must be replaced by calls to ``int``
-    * ``(int, long)`` for type-checking purposes must be replaced by
-      :py:data:`odoo.tools.pycompat.integer_types`
 
 
 * the ``L`` suffix on numbers is unsupported in Python 3, and unnecessary in
@@ -311,18 +309,6 @@ under the single ``int`` type.
   subtype of ``int`` (nor the reverse), and ``isinstance(value, (int, long))``
   is thus generally necessary to catch all integrals.
 
-  For that case, Odoo 11 now provides a compatibility module with an
-  :py:data:`~odoo.tools.pycompat.integer_types` definition which can be used
-  for type-testing.
-
-  It is a tuple of types so when used with ``isinstance`` it can be provided
-  directly or inside an other tuple alongside other types e.g.
-  ``isinstance(value, (BaseModel, integer_types))``.
-
-  However when used with ``type`` directly (which should be avoided) you
-  should use the ``in`` operator, and if you need other types you need to
-  concatenate ``integer_types`` to an other tuple.
-
 ``reduce``
 ##########
 
@@ -411,18 +397,6 @@ Both versions have the following prefixes for string literals:
 For best cross-version compatibility you should avoid unprefixed string
 literals unless you *specifically* need a "native string" [#native-string]_.
 
-For easier type-testing, :mod:`odoo.tools.pycompat` provides the following
-constants:
-
-* :data:`~odoo.tools.pycompat.string_types` is an alias/type tuple for testing
-  string types, essentially a replacement of testing for ``basestring`` or
-  ``(str, unicode)``.
-* :data:`~odoo.tools.pycompat.text_type` is the proper *text* type for the
-  current version, it should mostly be used for converting non-bytes objects
-  to text.
-* ``bytes`` should be avoided for type conversions, though it can be used to
-  check if an object is a bytestring.
-
 ``open``
 --------