[15.0][MIG] base_export_async: Migration to 15.0

[vincent-cowboy]

Inspired by #492

Help needed regarding RuntimeError("object unbound") when using xls format:

Traceback (most recent call last):
 (...)
  File "(...)/odoo15/oca/queue/base_export_async/models/delay_export.py", line 82, in export
    content = self._get_file_content(params)
  File "(...)/odoo15/oca/queue/base_export_async/models/delay_export.py", line 66, in _get_file_content
    return xlsx.from_data(columns_headers, import_data)
  File "(...)/odoo15/odoo/addons/web/controllers/main.py", line 1913, in from_data
    with ExportXlsxWriter(fields, len(rows)) as xlsx_writer:
  File "(...)/odoo15/odoo/addons/web/controllers/main.py", line 709, in __init__
    decimal_places = [res['decimal_places'] for res in request.env['res.currency'].search_read([], ['decimal_places'])]
  File "(...)/.pyenv/versions/odoo15/lib/python3.8/site-packages/werkzeug/local.py", line 348, in __getattr__
    return getattr(self._get_current_object(), name)
  File "(...)/.pyenv/versions/odoo15/lib/python3.8/site-packages/werkzeug/local.py", line 307, in _get_current_object
    return self.__local()
  File "(...)/.pyenv/versions/odoo15/lib/python3.8/site-packages/werkzeug/local.py", line 137, in _lookup
    raise RuntimeError("object unbound")
RuntimeError: object unbound