[FIX] requirements: use psycopg2 2.8.3 with Py3.8

When installing Odoo dependencies (from `requirements.txt`)
in Python 3.8, the installatio process of psycopg2 2.7.3.1 fails.

closes odoo/odoo#64612

Signed-off-by: Christophe Monniez (moc) <moc@odoo.com>

requirements.txt

@@ -24,8 +24,8 @@ Pillow==4.0.0 ; python_version < '3.7'
 Pillow==6.1.0 ; python_version >= '3.7'
 psutil==4.3.1; sys_platform != 'win32'
 psutil==5.6.3; sys_platform == 'win32'
-psycopg2==2.7.3.1; sys_platform != 'win32'
-psycopg2==2.8.3; sys_platform == 'win32'
+psycopg2==2.7.3.1; sys_platform != 'win32' and python_version < '3.8'
+psycopg2==2.8.3; sys_platform == 'win32' or python_version >= '3.8'
 pydot==1.2.3
 pyldap==2.4.28; sys_platform != 'win32'
 pyparsing==2.1.10