Disable Python 3's exception chaining when reraising

gym/utils/reraise.py

@@ -1,7 +1,7 @@
 import sys
 
-# We keep the actual reraising in different modules since the
-# Python 2 version SyntaxError's in Python 3.
+# We keep the actual reraising in different modules, since the
+# reraising code uses syntax mutually exclusive to Python 2/3.
 if sys.version_info[0] < 3:
     from .reraise_impl_py2 import reraise_impl
 else:

gym/utils/reraise_impl_py3.py

@@ -1,2 +1,4 @@
+# http://stackoverflow.com/a/33822606 -- `from None` disables Python 3'
+# semi-smart exception chaining, which we don't want in this case.
 def reraise_impl(e, traceback):
-    raise e.with_traceback(traceback)
+    raise e.with_traceback(traceback) from None