This is the latest version that still support 3.7.x. The 8.x series initially requires 3.8 and later
3.9. We could perhaps update to a 3.8-requiring version since this isn't used anywhere in automation,
but this version is a conservative choice for now.
Differential Revision: https://phabricator.services.mozilla.com/D194052
This is needed for a few reasons:
* All mach commands can use virtualenvs, not just `build`-related
commands, so the files don't make sense to be in `build/`.
* When locking is added, more files associated with virtualenvs will be
added, and this will change will ease the related directory structure
setup.
* This removes the need for a redundant "_virtualenv_packages" keyword
as part of the manifest filenames.
Differential Revision: https://phabricator.services.mozilla.com/D140382