Tests fail because names of pytest executable are hardcoded

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It happens because of this:

https://github.com/rags/pynt/blob/c0fbbc4c0c6c1ccbbf11a744fb62d33c78ac0ae0/build.py#L18

In my system, I have both Python 2 and 3 (maybe it's worth dropping Python 2 support already?). Executables are named py.test and py.test3.

To maintain backwards compatibility, it's probably worth wrapping those subprocess calls and if they fail - try generic names.

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Attempted to fix it in https://github.com/rags/pynt/pull/26