Bump `cryptography` python module version

The requirements.txt mandates 2.4.x version of cryptography python
module which has an issue as it checks if the payload is an instance
of bytes which is actually a bytearray. This issue is fixed in 2.5
version, hence bump up minimum version to 2.5.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>

requirements.txt

@@ -26,5 +26,5 @@ cmsis-pack-manager>=0.2.3,<0.3.0
 pywin32==224;platform_system=='Windows'
 wmi==1.4.9;platform_system=='Windows'
 psutil==5.6.2
-cryptography>=2.4.x,<2.5
+cryptography>=2.5,<3
 Click>=7.0,<7.1