AppVeyor Windows CI builds are failing

[exarkun]

%PYTHON%\Scripts\tox.exe -e py
Traceback (most recent call last):
  File "c:\python27\lib\runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "c:\python27\lib\runpy.py", line 72, in _run_code
    exec code in run_globals
  File "C:\Python27\Scripts\tox.exe\__main__.py", line 9, in <module>
  File "c:\python27\lib\site-packages\tox\session.py", line 47, in cmdline
    main(args)
  File "c:\python27\lib\site-packages\tox\session.py", line 52, in main
    config = prepare(args)
  File "c:\python27\lib\site-packages\tox\session.py", line 34, in prepare
    config = parseconfig(args)
  File "c:\python27\lib\site-packages\tox\config\__init__.py", line 238, in parseconfig
    ParseIni(config, config_file, content)
  File "c:\python27\lib\site-packages\tox\config\__init__.py", line 1070, in __init__
    config.envconfigs[name] = self.make_envconfig(name, section, reader._subs, config)
  File "c:\python27\lib\site-packages\tox\config\__init__.py", line 1141, in make_envconfig
    res = env_attr.postprocess(testenv_config=tc, value=res)
  File "c:\python27\lib\site-packages\tox\config\__init__.py", line 568, in basepython_default
    implied_version = tox.PYTHON.PY_FACTORS_RE.match(factor).group(2)
AttributeError: 'NoneType' object has no attribute 'group'

​https://ci.appveyor.com/project/tahoe-lafs/tahoe-lafs/builds/21835427/job/ka3oyne1svibilar

[GitHub <noreply@…>]

In d3b28e1/trunk:

Merge pull request #534 from tahoe-lafs/2956.appveyor-tox-errors

Fix the AppVeyor? CI job

This works around a Tox bug that prevents the job from even starting to run any tests.

Fixes ticket:2956