Consider whitelisting PROCESSOR_ARCHITECTURE environment variable

[asottile]

On windows, this is necessary for platform.machine() to function

$ tox --version
2.9.1 imported from c:\users\anthony\appdata\local\temp\venv\lib\site-packages\tox\__init__.pyc

[tox]
envlist = py27
skipsdist = true

[testenv]
commands =
    python -c 'import platform; print(platform.machine())'

$ tox -e py27
py27 create: C:\Users\Anthony\AppData\Local\Temp\.tox\py27
py27 installed:
py27 runtests: PYTHONHASHSEED='154'
py27 runtests: commands[0] | python -c import platform; print(platform.machine())

______________________________________________________ summary _______________________________________________________
  py27: commands succeeded
  congratulations :)

$ TOX_TESTENV_PASSENV=PROCESSOR_ARCHITECTURE tox -e py27
py27 installed:
py27 runtests: PYTHONHASHSEED='864'
py27 runtests: commands[0] | python -c import platform; print(platform.machine())
AMD64
______________________________________________________ summary _______________________________________________________
  py27: commands succeeded
  congratulations :)

Hitting this in pre-commit/pre-commit#685 where I'm working around this by adding PROCESSOR_ARCHITECTURE to passenv