[FIX] base: fix qweb ast validation for python 3.8.4

As of Python 3.8.4, when `ast.Name` is instantiated with either `True`,
`False` and `None`, a ValueError is raised [1][2].
Because of that, QWeb views processing will crash.

Replacing `ast.Name` with `ast.Constant` is the proper way to
instantiate those constants, and is supported since Python 3.6.0.

This fix was tested on Python 3.6, 3.7 and 3.8, and is in 14.0 already.

It is now considered for backport in 13.0 because:
- 13.0 still represented 75% of the new installations as of
  September 2020, so if left unpatched it will generate a large number
  of obscure errors and support tickets for our users.
- Ubuntu 20.04 has started to deploy Python 3.8.5 via unattended
  upgrades in October 2020, replacing 3.8.2, and thus triggers the
  problem [4].

1: https://docs.python.org/3/whatsnew/changelog.html#id7
2: https://bugs.python.org/issue40870
3: https://packages.ubuntu.com/focal/python3.8

backport of d73b44a using ast.NameConstant (available in python with
python/cpython@442f209
and deprecated in python 3.8) because python 3.5 doesn't have ast.Constant.
closes odoo#59630

Signed-off-by: Olivier Dony (odo) <[email protected]>

odoo/addons/base/models/ir_qweb.py

@@ -431,10 +431,10 @@ def _compile_expr(self, expr):
     def _get_attr_bool(self, attr, default=False):
         if attr:
             if attr is True:
-                return ast.Name(id='True', ctx=ast.Load())
+                return ast.NameConstant(True)
             attr = attr.lower()
             if attr in ('false', '0'):
-                return ast.Name(id='False', ctx=ast.Load())
+                return ast.NameConstant(False)
             elif attr in ('true', '1'):
-                return ast.Name(id='True', ctx=ast.Load())
-        return ast.Name(id=str(attr if attr is False else default), ctx=ast.Load())
+                return ast.NameConstant(True)
+        return ast.NameConstant(attr if attr is False else bool(default))

odoo/addons/base/models/qweb.py

@@ -613,12 +613,12 @@ def _if_content_is_not_Falsy(self, body, orelse):
                         ast.Compare(
                             left=ast.Name(id='content', ctx=ast.Load()),
                             ops=[ast.IsNot()],
-                            comparators=[ast.Name(id='None', ctx=ast.Load())]
+                            comparators=[ast.NameConstant(None)]
                         ),
                         ast.Compare(
                             left=ast.Name(id='content', ctx=ast.Load()),
                             ops=[ast.IsNot()],
-                            comparators=[ast.Name(id='False', ctx=ast.Load())]
+                            comparators=[ast.NameConstant(False)]
                         )
                     ]
                 ),
@@ -1246,7 +1246,7 @@ def _compile_widget(self, el, expression, field_options):
                         keywords=[], starargs=None, kwargs=None
                     ),
                     self._compile_expr0(expression),
-                    ast.Name(id='None', ctx=ast.Load()),
+                    ast.NameConstant(None),
                 ], ctx=ast.Load())
             )
         ]
@@ -1563,7 +1563,7 @@ def _compile_directive_call(self, el, options):
                     if isinstance(key, pycompat.string_types):
                         keys.append(ast.Str(s=key))
                     elif key is None:
-                        keys.append(ast.Name(id='None', ctx=ast.Load()))
+                        keys.append(ast.NameConstant(None))
                     values.append(ast.Str(s=value))
 
                 # {'nsmap': {None: 'xmlns def'}}