title: Tox: per-platform configuration for tools like flake8 and pylint
body: <p>I've got tox.ini configuration for ci both in linux and windows environments, something like that:</p>
  <pre><code>[tox]
  envlist =
      {py3,py27}-{test,lint}-{linux,windows}
      flake8
      check-package
  skip_missing_interpreters = true
  isolated_build = True
  distdir = dist
  
  [testenv]
  platform =
      linux: linux
      windows: win
  
  # Reuse py3 and py27 envs for pylint
  envdir =
      py3: {toxworkdir}/py3
      py27: {toxworkdir}/py27
      !py3-!py27: {toxworkdir}/{envname}
  deps =
      py27: setuptools &lt; 45.0.0
  
  # test extras must include pytest and pylint
  extras = test
  commands =
      test: python -m pytest -c {toxinidir}/pytest.ini --junitxml=test-reports/pytest.{envname}.xml {posargs}
      lint: python -m pylint --rcfile=tox.ini src/displaylink {posargs}
  
  [testenv:flake8]
  basepython = python3
  skip_install = true
  deps = flake8
  commands = flake8 src tests
  
  [tool:pylint]
  disable = missing-docstring,
            R,
            C,
            line-too-long
  output-format = parseable
  reports = no
  extension-pkg-whitelist = win32api, win32gui
  
  [flake8]
  ignore = E501, E722, W503
  per-file-ignores =
      # imported but unused
      __init__.py: F401
  max-complexity = 10
  </code></pre>
  <p>Question is how may I add per-platform configuration for tools (flake and pylint) ? I need to exclude files/dirs from flake8 and pylint runs depending on os, i.e. I've got windows subdir with files that won't pass linting on linux and vice-versa</p>
  <p>EDIT:</p>
  <p>example of pylint errors I have on linux:</p>
  <pre><code>py3-lint-linux run-test: commands[0] | python -m pylint --rcfile=tox.ini src/displaylink
  
  ************* Module displaylink.qa.windows.registry
  registry.py:4: [E0401(import-error), ] Unable to import 'win32con'
  registry.py:74: [E0602(undefined-variable), reg_value_exists] Undefined variable 'WindowsError'
  registry.py:82: [E0602(undefined-variable), reg_key_exists] Undefined variable 'WindowsError'
  </code></pre>