Duplicate pytest requirements in setup.py

[ruiqianyu]

pytest is required twice in setup.py with different versions. https://github.com/Thriftpy/thriftpy2/blob/master/setup.py#L36 https://github.com/Thriftpy/thriftpy2/blob/master/setup.py#L39

Seems to be the result of a recent branch merge https://github.com/Thriftpy/thriftpy2/commit/21b27cd1cc6a504072786130a74b36b7996ff2c2#diff-60f61ab7a8d1910d86d9fda2261620314edcae5894d5aaa236b821c7256badd7R39.

Remove "pytest>=2.8",?

[ethe]

Thanks, I will fix it later.