imgtool: Bump cryptography library version

Update requirements.txt to fix failing CI jobs due to
missing PrivateKeyType and PublicKeyType which are available
in cryptography library since version 40.0.0

Signed-off-by: Rustam Ismayilov <rustam.ismayilov@arm.com>
Change-Id: I63552966051a19dc33d5c53b499d216bcfdcf89b

scripts/requirements.txt

@@ -1,4 +1,4 @@
-cryptography>=2.6
+cryptography>=40.0.0
 intelhex
 click
 cbor2