[FIX] base: add compatiblity with python 3.8

Partial backport of aeb1e59 to 11.0

Without this commit, trying to access a QWeb page (e.g. the login
page), was failing during the compilation as missing the argument
posonlyargs.

Traceback (most recent call last):
  File ".../odoo/addons/base/ir/ir_qweb/qweb.py", line 329, in compile
    unsafe_eval(compile(astmod, '<template>', 'exec'), ns)
TypeError: required field "posonlyargs" missing from arguments

Error when compiling AST
TypeError: required field "posonlyargs" missing from arguments

ast.arguments does no validation but the compilation do

closes #40422

X-original-commit: 04893ed
Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>

odoo/addons/base/models/qweb.py

@@ -109,6 +109,7 @@ def visit_Lambda(self, node):
                 # handle that cross-version
                 kwonlyargs=[],
                 kw_defaults=[],
+                posonlyargs=[],
             ),
             body=Contextifier(self._safe_names + tuple(names)).visit(node.body)
         ), node)
@@ -554,7 +555,7 @@ def _create_def(self, options, body, prefix='fn', lineno=None):
                 arg(arg='values', annotation=None),
                 arg(arg='options', annotation=None),
                 arg(arg='log', annotation=None),
-            ], defaults=[], vararg=None, kwarg=None, kwonlyargs=[], kw_defaults=[]),
+            ], defaults=[], vararg=None, kwarg=None, posonlyargs=[], kwonlyargs=[], kw_defaults=[]),
             body=body or [ast.Return()],
             decorator_list=[])
         if lineno is not None: