[FIX] package: requirements for python 3.8

psycopg2 2.7 not be installed on python 3.8, needs at least psycopg2 2.8
use the same version as windows to avoid complicated rules if windows
AND python 3.8
Note that psycopg2 3.8.4 is currently the only one released after the
release of python 3.8 but reported compatibilty issued seems to be
fixed since 3.8 at psycopg/psycopg2#854

Fixes odoo/odoo#42660

closes odoo/odoo#42705

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>

requirements.txt

@@ -25,8 +25,8 @@ Pillow==5.4.1 ; python_version < '3.7' or sys_platform != 'win32'
 Pillow==6.1.0 ; sys_platform == 'win32' and python_version >= '3.7'
 polib==1.1.0
 psutil==5.5.1
-psycopg2==2.7.7; sys_platform != 'win32'
-psycopg2==2.8.3; sys_platform == 'win32'
+psycopg2==2.7.7; sys_platform != 'win32' and python_version < '3.8'
+psycopg2==2.8.3; sys_platform == 'win32' or python_version >= '3.8'
 pydot==1.4.1
 python-ldap==3.1.0; sys_platform != 'win32'
 pyparsing==2.2.0