[#8530] convert exception handlers to except..as syntax

Futurize fixers:
* lib2to3.fixes.fix_except

doc/python3.rst

@@ -0,0 +1,88 @@
+:orphan:
+
+==================================
+Python 3 compatibility/conversions
+==================================
+
+Goal (notsure?): for v11 to provide an alpha/beta Python 3 compatibility, for
+v12 to provide official Python 3 support and drop Python 2 in either v12 or
+v13.
+
+Python 2 and Python 3 are somewhat different language, but following
+backports, forward ports and cross-compatibility library it is possible to
+use a subset of Python 2 and Python 3 in order to have a system compatible
+with both.
+
+Here are a few useful steps or reminders to make Python 2 code compatible
+with Python 3.
+
+References/useful documents:
+
+* `How do I port to Python 3? <https://eev.ee/blog/2016/07/31/python-faq-how-do-i-port-to-python-3/>`_
+* `Python-Future <http://python-future.org/index.html>`_
+* `Porting Python 2 code to Python 3 <https://docs.python.org/3/howto/pyporting.html>`_
+* `Porting to Python 3: A Guide <http://lucumr.pocoo.org/2010/2/11/porting-to-python-3-a-guide/>`_ (a bit outdated but useful for the extensive comments on strings and IO)
+
+Versions Support
+================
+
+A cross compatible Odoo would only support Python 2.7 and Python 3.5 and
+above: Python 2.7 backported some Python 3 features, and Python 2 features
+were reintroduced in various Python 3 in order to make conversion easier.
+Python 3.6 adds great features (f-strings, ...) and performance improvements
+(ordered compact dicts) but does not seem to reintroduce compatibility
+features whereas:
+
+* Python 3.5 reintroduced ``%`` for bytes/bytestrings (:pep:`461`)
+* Python 3.4 has no specific compatibility improvement but is the lowest P3
+  version for PyLint
+* Python 3.3 reintroduced the "u" prefix for proper (unicode) strings
+* Python 3.2 made ``range`` views more list-like and reintroduced ``callable``
+
+.. warning::
+
+    Python 3 adds plenty of great features (keyword-only parameters,
+    generator delegation, pathlib, ...), do not use them until Python 2
+    support is dropped
+
+Fixes
+=====
+
+Exception Handlers
+------------------
+
+.. important::
+
+    All exception handlers must be converted to ``except ... as ..``. Valid
+    forms are::
+
+        except Exception:
+        except (Exception1, ...):
+        except Exception as name:
+        except (Exception1, ...) as name:
+
+In Python 2, ``except`` statements are of the form::
+
+    except Exception[, name]:
+
+or::
+
+    except (Exception1, Exception2)[, name]:
+
+But because the name is optional, this gets confusing and people can stumble
+into the first form when trying for the second and write::
+
+    except Exception1, Exception:
+
+which will *not* yield the expected result.
+
+Python 3 changes this syntax to::
+
+    except Exception[ as name]:
+
+or::
+
+    except (Exception1, Exception2)[ as name]:
+
+This form was implemented in Python 2.5 and is thus compatible across the
+board.