tox.ini: Bring over changes from 1.0-alpha branch
tox.ini: Bring over changes from 1.0-alpha branch

These are the changes that @sigmavirus24 did to `tox.ini` on the
`1.0-alpha` branch in
https://github.com/sigmavirus24/github3.py/pull/287

jim-minter/github3.py,h4ck3rm1k3/github3.py,itsmemattchung/github3.py,wbrefvem/github3.py,ueg1990/github3.py,sigmavirus24/github3.py,krxsky/github3.py,agamdua/github3.py,icio/github3.py,degustaf/github3.py,balloob/github3.py,christophelec/github3.py