Improve namedtuple definition (#2506)

While working on python/mypy#11206 I found that `Regspec` definition is not ideal.
It is recommended to use the same string name, as variable name. 
For example, it affects how `pickle` works.

Related #2501
Related 6e036f3

manticore/native/cpu/x86.py

@@ -144,7 +144,7 @@ def new_method(cpu, *args, **kw_args):
 
 
 class AMD64RegFile(RegisterFile):
-    Regspec = collections.namedtuple("RegSpec", "register_id ty offset size reset")
+    Regspec = collections.namedtuple("Regspec", "register_id ty offset size reset")
     _flags = {"CF": 0, "PF": 2, "AF": 4, "ZF": 6, "SF": 7, "IF": 9, "DF": 10, "OF": 11}
     _table = {
         "CS": Regspec("CS", int, 0, 16, False),