Fix none config file issue 3611

[kurtmckee]

This change fixes an issue that manifests when a user's default config file (like ~/.config/tox/config.ini) is corrupt: the phrase "failed to read config file None" appears in the output.

ERROR:root:failed to read config file None because File contains no section headers.
file: '/home/kurt/.config/tox/config.ini', line: 1
'[tox\n'

Fixes #3611

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation