[FIX] requirements: avoid double requirements for pillow

When installing requirements on MS Windows platform with Python 3.8, the
Pillow requirement is defined two times. This leads to a pip crash.

With this commit, the Pillow requirement is only defined once.

Fixes #40080

closes odoo/odoo#40239

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

requirements.txt

@@ -21,7 +21,7 @@ mock==2.0.0
 num2words==0.5.6
 ofxparse==0.16
 passlib==1.6.5
-Pillow==5.4.1
+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==4.3.1; sys_platform != 'win32'