Consolidate py2 and py3 requirements files

Consolidate py2 and py3 requirements files

Consolidate all py2 and py3 requirements into a single requirements.txt
file for each and pip install them in a single `pip install`. The
problem with running multiple pip installs separately is that Python
dependency resolution will not take into account previous pip installs
and can break package dependencies. Crafting a requirements.txt and
installing all packages at the same time allows pip to ensure that
all packages are installed together with dependency versions that
statisfy all requirements.

Now that only Python 3 is supported we can combine everything into a
single requirements.txt file.

Change-Id: I020ac24da62301c2d99645d5adae9b45f642cb75
Signed-off-by: Thanh Ha <zxiiro@gmail.com>