[Checkins] [zopefoundation/Products.StandardCacheManagers] cd5cfb: Fix linting issue

[Jürgen Gmach]

  Branch: refs/heads/fix-lint
  Home:   https://github.com/zopefoundation/Products.StandardCacheManagers
  Commit: cd5cfb4f3d10de890bd2b4d20510d9561957cbb3
      https://github.com/zopefoundation/Products.StandardCacheManagers/commit/cd5cfb4f3d10de890bd2b4d20510d9561957cbb3
  Author: Jürgen Gmach <juergen.gmach at googlemail.com>
  Date:   2020-05-15 (Fri, 15 May 2020)

  Changed paths:
    M tox.ini

  Log Message:
  -----------
  Fix linting issue

... which arise with the new flake8 version.

`tests` is no longer a valid flake8 param, as it is no top level
directory.

As it is a subfolder of the `src` folder, it gets linted anyway.

modified:   tox.ini