Support integer boolean constants.

tests/parser-cases/constants.thrift

@@ -1,3 +1,5 @@
+const bool tbool = true
+const bool tboolint = 1
 const i16 int16 = 3
 const i32 int32 = 800
 const i64 int64 = 123456789

tests/test_parser.py

@@ -12,6 +12,8 @@ def test_comments():
 
 def test_constants():
     thrift = load('parser-cases/constants.thrift')
+    assert thrift.tbool is True
+    assert thrift.tboolint is True
     assert thrift.int16 == 3
     assert thrift.int32 == 800
     assert thrift.int64 == 123456789

thriftpy/parser/parser.py

@@ -601,8 +601,8 @@ def _cast(t):  # noqa
 
 
 def _cast_bool(v):
-    assert isinstance(v, bool)
-    return v
+    assert isinstance(v, (bool, int))
+    return bool(v)
 
 
 def _cast_byte(v):