Fix libMP compile issue: exec problem

Error during sympy setup:

byte-compiling /usr/local/lib/python2.7/dist-packages/sympy/mpmath/libmp/exec_py3.py to exec_py3.pyc
  File "/usr/local/lib/python2.7/dist-packages/sympy/mpmath/libmp/exec_py3.py", line 1
    exec_ = exec
               ^
SyntaxError: invalid syntax

fixed it by looking at:
http://code.google.com/p/mpmath/issues/detail?id=204