$ virtualenv .venv
created virtual environment CPython3.6.8.final.0-64 in 243ms
  creator CPython3Posix(dest=/tmp/grenade/.venv, clear=False, no_vcs_ignore=False, global=False)
  seeder FromAppData(download=False, pip=bundle, setuptools=bundle, wheel=bundle, via=copy, app_data_dir=/home/centos/.local/share/virtualenv)
    added seed packages: pip==21.0.1, setuptools==53.0.0, wheel==0.36.2
  activators BashActivator,CShellActivator,FishActivator,PowerShellActivator,PythonActivator,XonshActivator
$ source .venv/bin/activate
(.venv)$ 
(.venv)$ pip install tox==1.6
Collecting tox==1.6
  Downloading tox-1.6.0.tar.gz (130 kB)
     |████████████████████████████████| 130 kB 8.6 MB/s 
Collecting virtualenv>=1.9.1
  Downloading virtualenv-20.4.3-py2.py3-none-any.whl (7.2 MB)
     |████████████████████████████████| 7.2 MB 8.6 MB/s 
Collecting py>=1.4.15
  Using cached py-1.10.0-py2.py3-none-any.whl (97 kB)
Collecting appdirs<2,>=1.4.3
  Using cached appdirs-1.4.4-py2.py3-none-any.whl (9.6 kB)
Collecting importlib-resources>=1.0
  Using cached importlib_resources-5.1.2-py3-none-any.whl (25 kB)
Collecting six<2,>=1.9.0
  Using cached six-1.15.0-py2.py3-none-any.whl (10 kB)
Collecting filelock<4,>=3.0.0
  Using cached filelock-3.0.12-py3-none-any.whl (7.6 kB)
Collecting importlib-metadata>=0.12
  Downloading importlib_metadata-3.7.3-py3-none-any.whl (12 kB)
Collecting distlib<1,>=0.3.1
  Using cached distlib-0.3.1-py2.py3-none-any.whl (335 kB)
Collecting typing-extensions>=3.6.4
  Using cached typing_extensions-3.7.4.3-py3-none-any.whl (22 kB)
Collecting zipp>=0.5
  Using cached zipp-3.4.1-py3-none-any.whl (5.2 kB)
Building wheels for collected packages: tox
  Building wheel for tox (setup.py) ... done
  Created wheel for tox: filename=tox-1.6.0-py3-none-any.whl size=83392 sha256=91ed81c3f702f53062da0188f19b02b5939747f82bbd27307a07a54171022e62
  Stored in directory: /home/centos/.cache/pip/wheels/29/8e/6f/9c42a50c15020383a4da5cda84e3d3206b82aa4e4d6f25c3ab
Successfully built tox
Installing collected packages: zipp, typing-extensions, six, importlib-resources, importlib-metadata, filelock, distlib, appdirs, virtualenv, py, tox
Successfully installed appdirs-1.4.4 distlib-0.3.1 filelock-3.0.12 importlib-metadata-3.7.3 importlib-resources-5.1.2 py-1.10.0 six-1.15.0 tox-1.6.0 typing-extensions-3.7.4.3 virtualenv-20.4.3 zipp-3.4.1
(.venv)$ tox -edocs
Traceback (most recent call last):
  File "/tmp/grenade/.venv/bin/tox", line 8, in <module>
    sys.exit(cmdline())
  File "/tmp/grenade/.venv/lib/python3.6/site-packages/tox/_cmdline.py", line 25, in main
    config = parseconfig(args, 'tox')
  File "/tmp/grenade/.venv/lib/python3.6/site-packages/tox/_config.py", line 44, in parseconfig
    parseini(config, inipath)
  File "/tmp/grenade/.venv/lib/python3.6/site-packages/tox/_config.py", line 236, in __init__
    config)
  File "/tmp/grenade/.venv/lib/python3.6/site-packages/tox/_config.py", line 300, in _makeenvconfig
    for depline in reader.getlist(section, "deps"):
  File "/tmp/grenade/.venv/lib/python3.6/site-packages/tox/_config.py", line 413, in getlist
    s = self.getdefault(section, name, None)
  File "/tmp/grenade/.venv/lib/python3.6/site-packages/tox/_config.py", line 511, in getdefault
    x = self._replace(x)
  File "/tmp/grenade/.venv/lib/python3.6/site-packages/tox/_config.py", line 618, in _replace
    return rexpattern.sub(replace_func, x)
  File "/tmp/grenade/.venv/lib/python3.6/site-packages/tox/_config.py", line 609, in _replace_match_no_quote
    return self._replace_match(match, quote=False)
  File "/tmp/grenade/.venv/lib/python3.6/site-packages/tox/_config.py", line 604, in _replace_match
    return handler(match, quote)
  File "/tmp/grenade/.venv/lib/python3.6/site-packages/tox/_config.py", line 541, in _replace_env
    (envkey, envkey))
tox.ConfigError: ConfigError: substitution env:'TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master': unkown environment variable 'TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master'


Update to tox>=2.0 solves the issue.