Add Python requirements.txt

[MarcelRobitaille]

This pull request moves the list of python dependencies from install_dependencies.sh into requirements.txt. There are a few reasons to do this:

1. With the dependencies defined outside of the install script, it's possible to install them or easily read the list of dependencies with other tools without running the install script. For example, the reason I am making this PR is that in the PKGBUILD for the Arch User Repository package I maintain, I can just do pip install -r requirements.txt. Currently, I have pretty much the same 8 pip install lines that you have in install_dependencies.sh. If these lines are changed in kiri without my noticing, the AUR package would break.
2. Having a requirements.txt file (or setup.py file) is the Python standard
3. It should be faster to only invoke pip once with the full list of dependencies

[leoheck]

This pull request moves the list of python dependencies from install_dependencies.sh into requirements.txt. There are a few reasons to do this:

Ok.

1. Ok, I am fine with this one
2. I don't care
3. Nah, it is run just once.

Here is your requirements.txt

grep "yes | pip" install_dependencies.sh | cut -d"\"" -f2

I don't like this file because it uses a silly name. Not even a creative one. The name requirements.txt means nothing. If it was at least python_requirements.txt I will think it is ok.

Are you ok if we name this file python_requirements.txt?