This PR adds initial support for type hints checking in python scripts.

Support for type hints was introduced in Python 3.5. Type hints make it easier to read and review code in my opinion. Also an IDE may discover a potential bug sooner. Yet, as PEP 484 says: "It should also be emphasized that Python will remain a dynamically typed language, and the authors have no desire to ever make type hints mandatory, even by convention."

Mypy is used in lint-python.sh to do the type checking. The package is standard so there is little chance that it will be abandoned. Mypy checks that type hints in source code are correct when they are not, it fails with an error.

Useful resources:

* https://docs.python.org/3.5/library/typing.html
* https://www.python.org/dev/peps/pep-0484/

test/functional/test_framework/script.py

@@ -9,6 +9,7 @@ This file is modified from python-bitcoinlib.
 import hashlib
 import struct
 import unittest
+from typing import List, Dict
 
 from .messages import (
     CTransaction,
@@ -21,7 +22,7 @@ from .messages import (
 )
 
 MAX_SCRIPT_ELEMENT_SIZE = 520
-OPCODE_NAMES = {}
+OPCODE_NAMES = {}  # type: Dict[CScriptOp, str]
 
 def hash160(s):
     return hashlib.new('ripemd160', sha256(s)).digest()
@@ -37,7 +38,7 @@ def bn2vch(v):
     # Serialize to bytes
     return encoded_v.to_bytes(n_bytes, 'little')
 
-_opcode_instances = []
+_opcode_instances = []  # type: List[CScriptOp]
 class CScriptOp(int):
     """A single script opcode"""
     __slots__ = ()

test/functional/test_framework/test_framework.py

@@ -91,6 +91,9 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
 
     This class also contains various public and private helper methods."""
 
+    chain = None  # type: str
+    setup_clean_chain = None  # type: bool
+
     def __init__(self):
         """Sets test framework defaults. Do not override this method. Instead, override the set_test_params() method"""
         self.chain = 'regtest'
@@ -407,7 +410,7 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
 
     # Public helper methods. These can be accessed by the subclass test scripts.
 
-    def add_nodes(self, num_nodes, extra_args=None, *, rpchost=None, binary=None, binary_cli=None, versions=None):
+    def add_nodes(self, num_nodes: int, extra_args=None, *, rpchost=None, binary=None, binary_cli=None, versions=None):
         """Instantiate TestNode objects.
 
         Should only be called once after the nodes have been specified in