cross py2/py3 syntax fix

web3/exceptions.py

@@ -29,4 +29,4 @@ class BadFunctionCallOutput(Exception):
     """We failed to decode ABI output.
 
     Most likely ABI mismatch.
-    """
+    """

web3/utils/exception.py

@@ -1,16 +1,7 @@
 import sys
 
 # raise MyException() from original_exception compatibility
-if sys.version_info.major == 3:
-
-    def raise_from(my_exception, other_exception):
-        raise my_exception from other_exception
+if sys.version_info.major == 2:
+    from .exception_py2 import raise_from
 else:
-
-    def raise_from(my_exception, other_exception):
-        # This syntax is not accepted under Python 3
-        # raise my_exception, None, sys.exc_info()[2]
-
-        # For now, just swallow the original exception under Python 2.
-        # For better error messages please upgrade to Python 3.
-        raise my_exception
+    from .exception_py3 import raise_from

web3/utils/exception_py2.py

@@ -0,0 +1,2 @@
+def raise_from(my_exception, other_exception):
+    raise my_exception, None, sys.exc_info()[2]  # noqa [w602]

web3/utils/exception_py3.py

@@ -0,0 +1,2 @@
+def raise_from(my_exception, other_exception):
+    raise my_exception from other_exception