I can reproduce here (OS X 10.9, Python 3.5), exactly as described in the original post.

Python 3.5.0 (default, Sep 22 2015, 18:26:54) 
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)] on darwin
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>>> import statistics
>>> statistics.mean([8.988465674311579e+307, 8.98846567431158e+307])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/statistics.py", line 293, in mean
    return _sum(data)/n
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/statistics.py", line 184, in _sum
    return T(total)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/numbers.py", line 291, in __float__
    return self.numerator / self.denominator
OverflowError: integer division result too large for a float