F2F: Fix crushes of the contracts containing non-ascii characters

Challenge: https://www.topcoder.com/challenges/30066271
Submission: 525176
Author: N1k1tung

mythril/ether/soliditycontract.py

@@ -36,7 +36,7 @@ class SolidityContract(ETHContract):
         self.solidity_files = []
 
         for filename in data['sourceList']:
-            with open(filename, 'r') as file:
+            with open(filename, 'r', encoding='utf-8') as file:
                 code = file.read()
                 self.solidity_files.append(SolidityFile(filename, code))
 

tests/testdata/input_contracts/nonascii.sol

@@ -0,0 +1,6 @@
+pragma solidity ^0.4.22;
+contract nonAscii {
+  function renderNonAscii () public pure returns (string) {
+	  return "Хэллоу Ворлд";
+  }
+}