Prior to this commit:
* Our Python dependency story was a bit of a mess. We had complete
Python packages (wheels and directories) living in-tree, despite
not having any changes from upstream. This is particularly bad because
`setup.py` never gets run on these packages which could (sometimes
silently) unintended breakage.
* Python virtual environments (virtualenv) were only utilized for
testing web-platform tests
After this commit:
* A single virtualenv (`python/_virtualenv`) is activated upon *every*
call to mach
* A requirements file (`python/requirements.txt`) is added to describe
the dependencies needed by Python modules in `python/`. The child
commit immediately following this will remove all the dependencies
no longer needed in-tree (for the sake of keeping this commit
readable).
Relevant to https://github.com/servo/servo/issues/861
Fixes https://github.com/servo/servo/issues/6999
