Ignore plyvel (#1543)

mythril/ethereum/interface/leveldb/eth_db.py

@@ -1,13 +1,23 @@
 """This module contains the ETH_DB class, which the base database used by
 pyethereum."""
-import plyvel
+
 from ethereum.db import BaseDB
 
+has_plyvel = True
+try:
+    import plyvel
+except ImportError:
+    has_plyvel = False
+
 
 class ETH_DB(BaseDB):
     """Adopts pythereum BaseDB using plyvel."""
 
     def __init__(self, path):
+        if has_plyvel is False:
+            raise ImportError(
+                "Plyvel is not installed, please install plyvel and leveldb"
+            )
         self.db = plyvel.DB(path)
 
     def get(self, key):