update readme with python3 support about dronekit-sitl HOT 4 OPEN

Hi @mrpollo

Never used Pip3. Internet seems to indicate that pip, pip2, pip3 are just different revisions of the same program, which presumably in version pip3 knows to get Python 3 versions of the binaries. Some of the other resources out there seem to indicate that if you install pip3, then calling pip will get you that anyway - in other words, you get the pip that is appropriate for your current environment.

Is that correct? If so, then the following instructions should be python-version-agnostic right ? :

pip install dronekit-sitl -UI

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thats correct!, i would much rather go with just pip instead of pip2, pip3

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@mrpollo Agreed. Only issue is that this doesn't work for me on Windows. So action is to remove the "2" in pip2 commands in readme.md, but only after #68 is addressed.

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On Mac OSX, we use homebrew to install both Python2 and Python3 (and Pypy and Pypy3) on the same machine. The use of pip2 and pip3 (and pip_pypy and pip_pypy3) to enable users to install a module for one particular Python implementation and not for the others. These are the same pip binary but each is configured to put their installation results into different site-packages. I suspect that there are similar approaches on other platforms. I agree with @mrpollo above that each user needs to understand how pip is setup on their own machines.

$ pip -V
pip 8.1.2 from /usr/local/lib/python2.7/site-packages (python 2.7)
$ pip3 -V
pip 8.1.2 from /usr/local/lib/python3.6/site-packages (python 3.6)

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