RFC - tox changes (SC-763)

[holmanb]

fix parallel tox execution

Changes:

• tox -p fails due to a shared directory clobbering files - use unique env dirs to avoid this (this does cause a longer initial serial run due to deps getting installed multiple times)
• remove unneeded dependencies for black, flake8, and isort environments (saves some CI cycles by avoiding installing unused deps - since travis doesn't share a cache this should be a net gain)
• add a new environment (for convenience) just for "checking formatting" - this moves current "format" env to "do_format" and adds "check_format". I'm happy to adjust names if others would rather "format" doesn't change or think there is some better naming scheme.

[TheRealFalcon]

Overall approach looks good. I left some comments inline.

[TheRealFalcon]

Parallelize all the things!