Python3 Not Supported

[xanderdunn]

Python3.5 with tensorflow 0.10.0.

>>> import imperative
>>> imperative.self_test()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/local/dev/imperative/release/imperative.py", line 1721, in self_test
    env = Env(tf)
  File "/local/dev/imperative/release/imperative.py", line 1328, in __init__
    rewriter = ModuleRewriter(symbol_rewriter, "imperative.")
  File "/local/dev/imperative/release/imperative.py", line 978, in __init__
    for _unused_name, module in sys.modules.items():
RuntimeError: dictionary changed size during iteration

Supporting Python3 isn't very important, but wanted to document that it should only be used with Python2 for now.