[bug#29915] [PATCH] gnu: Add python-pyhamcrest.

[Fis Trivial]

> 
> Six would be a propagated input here, seeing that it is in the
> install_requires list in Hamcrest's setup.py file. For more details and
> future reference, this type of thing is in the "Python Modules" section
> of Guix's manual.
> 

For reasons I don't know(I am new to guix, sorry about that). I added six
because without it, the build phase failed on my device.

> 
> I've also added python2-hamcrest to the list of new variables in your
> commit message. I've made changed the bits I mentioned above and pushed
> your changes. Thanks for contributing!
> 

Thanks.