test: simplify uint256 (de)serialization routines

These routines look fancy, but do nothing more than converting between
byte objects of length 32 to/from integers in little endian byte order
and can be replaced by simple one-liners, using the int.{from,to}_bytes
methods (available since Python 3.2).

test/functional/test_framework/messages.py

@@ -93,6 +93,7 @@ def ser_compact_size(l):
         r = struct.pack("<BQ", 255, l)
     return r
 
+
 def deser_compact_size(f):
     nit = struct.unpack("<B", f.read(1))[0]
     if nit == 253:
@@ -103,35 +104,26 @@ def deser_compact_size(f):
         nit = struct.unpack("<Q", f.read(8))[0]
     return nit
 
+
 def deser_string(f):
     nit = deser_compact_size(f)
     return f.read(nit)
 
+
 def ser_string(s):
     return ser_compact_size(len(s)) + s
 
+
 def deser_uint256(f):
-    r = 0
+    return int.from_bytes(f.read(32), 'little')
-    for i in range(8):
-        t = struct.unpack("<I", f.read(4))[0]
-        r += t << (i * 32)
-    return r
 
 
 def ser_uint256(u):
-    rs = b""
+    return u.to_bytes(32, 'little')
-    for _ in range(8):
-        rs += struct.pack("<I", u & 0xFFFFFFFF)
-        u >>= 32
-    return rs
 
 
 def uint256_from_str(s):
-    r = 0
+    return int.from_bytes(s[:32], 'little')
-    t = struct.unpack("<IIIIIIII", s[:32])
-    for i in range(8):
-        r += t[i] << (i * 32)
-    return r
 
 
 def uint256_from_compact(c):