Merge #14884: Travis: enforce Python 3.4 support through linter

31926ee8cf [test] functional framework: add CScript hex() for Python 3.4 (Sjors Provoost)
74ce326831 [test] Travis: enforce Python 3.4 support in functional tests (Sjors Provoost)

Pull request description:

  The minimum supported version of Python is 3.4 according to [dependencies.md](https://github.com/bitcoin/bitcoin/blob/master/doc/dependencies.md). This PR makes the Travis linter use this version in order to catch accidental use of modern syntax.

Tree-SHA512: 71b2c102be72b135a8ba049378d66875760f20a04a657102a399240c5c2b2ddbdfa7d5ab4c0c0242ecc3259e0ee8eb2273f331bc5eb824f4ae4c3cc58aea37ac

test/functional/test_framework/script.py

@@ -450,6 +450,10 @@ class CScript(bytes):
         # join makes no sense for a CScript()
         raise NotImplementedError
 
+    # Python 3.4 compatibility
+    def hex(self):
+        return hexlify(self).decode('ascii')
+
     def __new__(cls, value=b''):
         if isinstance(value, bytes) or isinstance(value, bytearray):
             return super(CScript, cls).__new__(cls, value)