Use cryptographically secure generador for private keys (#118)

* Use cryptographically secure generador for pkeys

The random module should not be used for security purposes, therefore
the use of this module to generate private keys is a security flaw.

Since this project is not targeting an specific python version, I'm not
sure if it's wise to use the `secrets` module, because this module first
appeared on in python 3.6. Instead I'm using `_get_random_bytes()`
(which is os.urandom under the hood) to generate cryptographically
secure numbers.

On the other hand and I'm not sure if this logic of generating random
private keys is entirely secure. In my opinion, the right way of doing
this is using ECDSA or using the library we already have in this
project:

from Crypto.PublicKey import RSA
key = RSA.generate(2048)

private_key = key.export_key()
public_key = key.publickey().export_key()

* Use os.urandom directly

---------

Co-authored-by: MrNaif2018 <chuff184@gmail.com>

tronpy/keys/__init__.py

@@ -1,5 +1,5 @@
 import hashlib
-import random
+import os
 from collections.abc import ByteString, Hashable
 from typing import Any, Iterator, Union
 
@@ -274,7 +274,7 @@ def sign_msg_hash(self, message_hash: bytes) -> "Signature":
     @classmethod
     def random(cls) -> "PrivateKey":
         """Generate a random private key."""
-        return cls(bytes([random.randint(0, 255) for _ in range(32)]))
+        return cls(os.urandom(32))
 
     @classmethod
     def from_passphrase(cls, passphrase: bytes) -> "PrivateKey":