[IMP] web: improve eval context dates

This commit handles 2 things:

1) the merging of the `py_utils.context()` into the evaluation context
returned by `BasicModel._getEvalContext` to access the same time-related
keys,

2) the addition of 2 new keys into the `py_utils.context` (thus
transmitted to the basic model): `today` (an alias for the already
present `current_date`) and `now` which represents the current date and
time value.

Task 2269697

odoo/tools/view_validation.py

@@ -36,6 +36,8 @@ def _get_attrs_symbols():
         'datetime',
         'relativedelta',
         'current_date',
+        'today',
+        'now',
         'abs',
         'len',
         'bool',