automation: create Python 3.5 variant of requirements.txt
The automation environment is refusing to build with the previous
file because some dependencies won't install on Python 3.5.
I couldn't find an easy way to salvage the situation with a single
requirements.txt file. So, I decided to introduce a variant
for Python 3.5.
As part of this, we update packages to latest versions.
(I do question why we are still supporting Python 3.5...)
Differential Revision: https://phab.mercurial-scm.org/D10690