We use this option in the custodia package.

With python-tox 2.1.1 it breaks, reverting back to 1.8.1 works.

Here is the traceback:
Traceback (most recent call last):
  File "/usr/bin/tox", line 9, in <module>
    load_entry_point('tox==2.1.1', 'console_scripts', 'tox')()
  File "/usr/lib/python2.7/site-packages/tox/session.py", line 38, in main
    config = prepare(args)
  File "/usr/lib/python2.7/site-packages/tox/session.py", line 26, in prepare
    config = parseconfig(args)
  File "/usr/lib/python2.7/site-packages/tox/config.py", line 219, in parseconfig
    parseini(config, inipath)
  File "/usr/lib/python2.7/site-packages/tox/config.py", line 669, in __init__
    self.make_envconfig(name, section, reader._subs, config)
  File "/usr/lib/python2.7/site-packages/tox/config.py", line 697, in make_envconfig
    res = meth(env_attr.name, env_attr.default)
  File "/usr/lib/python2.7/site-packages/tox/config.py", line 832, in getdict
    s = self.getstring(name, None)
  File "/usr/lib/python2.7/site-packages/tox/config.py", line 886, in getstring
    x = self._replace(x)
  File "/usr/lib/python2.7/site-packages/tox/config.py", line 982, in _replace
    return RE_ITEM_REF.sub(self._replace_match, x)
  File "/usr/lib/python2.7/site-packages/tox/config.py", line 978, in _replace_match
    return handler(match)
  File "/usr/lib/python2.7/site-packages/tox/config.py", line 948, in _replace_substitution
    val = self._substitute_from_other_section(sub_key)
  File "/usr/lib/python2.7/site-packages/tox/config.py", line 942, in _substitute_from_other_section
    "substitution key %r not found" % key)
tox.ConfigError: ConfigError: substitution key 'envsitepackagesdir' not found