[thread-cert] import Crypto on demand (#8109)

This commit imports `Crypto` module only when it's used.

This helps thread-cert tests that do not use message factory to run
without having to install `Crypto` module.

tests/scripts/thread-cert/net_crypto.py

@@ -33,8 +33,6 @@ import struct
 
 from binascii import hexlify
 
-from Crypto.Cipher import AES
-
 
 class CryptoEngine:
     """ Class responsible for encryption and decryption of data. """
@@ -64,6 +62,8 @@ class CryptoEngine:
         """
         key, nonce, auth_data = self._crypto_material_creator.create_key_and_nonce_and_authenticated_data(message_info)
 
+        from Crypto.Cipher import AES
+
         cipher = AES.new(key, AES.MODE_CCM, nonce, mac_len=self.mic_length)
         cipher.update(auth_data)
 
@@ -83,6 +83,8 @@ class CryptoEngine:
         """
         key, nonce, auth_data = self._crypto_material_creator.create_key_and_nonce_and_authenticated_data(message_info)
 
+        from Crypto.Cipher import AES
+
         cipher = AES.new(key, AES.MODE_CCM, nonce, mac_len=self.mic_length)
         cipher.update(auth_data)