[FIX] report_py3o: Use the right naming convention for the render method

To determine the method to use to render a recport according to its type, the generic method 'render' defined into ir.action.report check if a method name  is defined on the model. https://github.com/odoo/odoo/blob/12.0/odoo/addons/base/models/ir_actions_report.py#L734
Therefore, we must provide this method to be compliant with what's expected by Odoo.
This change will also allows the usage of py3o template with mail_template once https://github.com/odoo/odoo/pull/30013 will be merged

report_py3o/controllers/main.py

@@ -44,7 +44,7 @@ class ReportController(main.ReportController):
             raise exceptions.HTTPException(
                 description='Py3o action report not found for report_name '
                             '%s' % reportname)
-        res, filetype = action_py3o_report._render_py3o(docids, data)
+        res, filetype = action_py3o_report.render_py3o(docids, data)
         filename = action_py3o_report.gen_report_download_filename(
             docids, data)
         if not filename.endswith(filetype):

report_py3o/models/ir_actions_report.py

@@ -81,12 +81,12 @@ class IrActionsReport(models.Model):
     def render_report(self, res_ids, name, data):
         action_py3o_report = self.get_from_report_name(name, "py3o")
         if action_py3o_report:
-            return action_py3o_report._render_py3o(res_ids, data)
+            return action_py3o_report.render_py3o(res_ids, data)
         return super(IrActionsReport, self).render_report(
             res_ids, name, data)
 
     @api.multi
-    def _render_py3o(self, res_ids, data):
+    def render_py3o(self, res_ids, data):
         self.ensure_one()
         if self.report_type != "py3o":
             raise RuntimeError(