[FIX] requirements: limit the version of cryptography

It happens that pyopenssl==19.0.0 is incompatible with cryptography>=37.0.0, but in pyopenssl requirements, they have lower bound limit for cryptography but not an upper bound.

closes odoo/odoo#99829

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

requirements.txt

 Babel==2.9.1  # min version = 2.6.0 (Focal with security backports)
 chardet==3.0.4
+cryptography==2.6.1  # incompatibility between pyopenssl 19.0.0 and cryptography>=37.0.0
 decorator==4.4.2
 docutils==0.16
 ebaysdk==2.1.5