Remove unused install_requires parameter in setup.py.

Also, renamed PyTest class to PyTestCommand and made
a couple cosmetic changes.
Python 3.1 is not supported anymore.
Merge pull request #914 from berkerpeksag/remove-3.1-support

Python 3.1 is not supported anymore.
Merge pull request #913 from berkerpeksag/tweak-setup

Remove unused install_requires parameter in setup.py.