[IMP] convert: autofill sequence fields

Motivations:
* `sequence` is not super intuitive for new devs
* It makes "useless" noise in data files
* It is redundant when what we want to see in the file is the same as
  what we want to see on the user screen.
Do you understand anything related to sequences in here?
https://github.com/odoo/odoo/blob/15.0/addons/l10n_lu/data/account_tax_report_line.xml
Explicit is not always better than implicit.
* It is not implicit like using `id`  it is natural and visual and saves
  a lot of chore for adding and maintaining sequences.

Implementation
* It is optional, with the option set on root xml tags

Part-of: odoo/odoo#85750

odoo/import_xml.rng

@@ -314,6 +314,7 @@
                 <rng:name>data</rng:name>
             </rng:choice>
             <rng:optional><rng:attribute name="noupdate" /></rng:optional>
+            <rng:optional><rng:attribute name="auto_sequence" /></rng:optional>
             <rng:ref name="env"/>
             <rng:zeroOrMore>
                 <rng:choice>

odoo/tools/convert.py

@@ -229,7 +229,7 @@ class xml_import(object):
                     **safe_eval(context, {
                         'ref': self.id_get,
                         **(eval_context or {})
-                    })
+                    }),
                 }
             )
         return self.env
@@ -446,6 +446,10 @@ form: module.record_id""" % (xml_id,)
             res[f_name] = f_val
         if extra_vals:
             res.update(extra_vals)
+        if 'sequence' not in res and 'sequence' in model._fields:
+            sequence = self.next_sequence()
+            if sequence:
+                res['sequence'] = sequence
 
         data = dict(xml_id=xid, values=res, noupdate=self.noupdate)
         record = model._load_records([data], self.mode == 'update')
@@ -546,6 +550,7 @@ form: module.record_id""" % (xml_id,)
 
             self.envs.append(self.get_env(el))
             self._noupdate.append(nodeattr2bool(el, 'noupdate', self.noupdate))
+            self._sequences.append(0 if nodeattr2bool(el, 'auto_sequence', False) else None)
             try:
                 f(rec)
             except ParseError:
@@ -568,6 +573,7 @@ form: module.record_id""" % (xml_id,)
             finally:
                 self._noupdate.pop()
                 self.envs.pop()
+                self._sequences.pop()
 
     @property
     def env(self):
@@ -577,12 +583,19 @@ form: module.record_id""" % (xml_id,)
     def noupdate(self):
         return self._noupdate[-1]
 
+    def next_sequence(self):
+        value = self._sequences[-1]
+        if value is not None:
+            value = self._sequences[-1] = value + 10
+        return value
+
     def __init__(self, env, module, idref, mode, noupdate=False, xml_filename=None):
         self.mode = mode
         self.module = module
         self.envs = [env(context=dict(env.context, lang=None))]
         self.idref = {} if idref is None else idref
         self._noupdate = [noupdate]
+        self._sequences = []
         self.xml_filename = xml_filename
         self._tags = {
             'record': self._tag_record,