Switch from pycypto to pycryptodomex

I suspect this might fix LEARNER-3705 because pycrypto was removed in
edx-platform by https://github.com/edx/edx-platform/pull/16612.

Regardless, PyCryptodome is more maintained than PyCrypto.

edx_proctoring/backends/software_secure.py

@@ -17,7 +17,7 @@ import requests
 
 from django.conf import settings
 
-from Crypto.Cipher import DES3
+from Cryptodome.Cipher import DES3
 
 from edx_proctoring.backends.backend import ProctoringBackendProvider
 from edx_proctoring import constants

requirements/base.in

@@ -6,7 +6,7 @@ django-model-utils>=2.3.1
 djangorestframework>=3.1,<3.7
 django-ipware>=1.1.0
 pytz>=2012h
-pycrypto>=2.6
+pycryptodomex>=3.4.7
 python-dateutil>=2.1
 
 # edX packages