[doc] new behavior in virtualenv since 3.26

[skeleton] use python3 flake8 In tox "flake8" environment, use python 3 virtualenv, so we can check for python3 incompatible syntax.

[skeleton] drop whitelist_externals for flake8 whitelist_externals makes sense when used with sitepackages=true. In this case it will always rely on $VIRTUAL_ENV/bin/flake8 so, the whitelist_externals has no effect.