[FIX] {sale_}purchase{_stock}: select sellers depending on date

Commit 72a17baf force the PO `date_order` to be >= at ` today() to
ensure the select_seller method take the right seller price. This
introduces an error in the MPS module that may need to create PO in the
past.

Instead, this commit patch the call to `_select_seller` to take at least
`today()`

closes odoo/odoo#133759

Opw: 3167094
Related: odoo/enterprise#46623
Signed-off-by: Arnold Moyaux (arm) <arm@odoo.com>