Remove vestigial `whitelist_externals` from Tox configuration

[sajith]

There are a few stray whitelist_externals = mv directives left in tox.ini:

[testenv:codechecks3]
basepython = python3
...
whitelist_externals =
         /bin/mv
...
[testenv:draftnews]
...
whitelist_externals = mv
...
[testenv:news]
...
whitelist_externals =
    mv
...

This was needed in older revisions of tox.ini (prior to fixing #3730), when towncrier configuration file used to be renamed inside the environment (with mv towncrier.pyproject.toml pyproject.toml) prior to running towncrier, and then renamed back (with mv pyproject.toml towncrier.pyproject.toml).