Allow braced range syntax in internal sections of tox.ini file

[marcosboger]

Closes #3571
Partially fixes #3614 -> This PR covers the complaint about using dash syntax in deps. It does not address the complaint about nested braces syntax.

This PR refactors how expand_ranges is used when loading .ini config files. Instead of calling expand_ranges directly when expanding envs or section names, we expand it once we start finding the factor groups, in find_factor_groups() function.

This means that ranges in sections like deps and setenv will also be expanded. Before, only env_list and section names were expanded. Considered as bugfix due to the original issue classification, but could also arguably be considered a small feature.

The approach was done in this way so that we don't have to "remember" to call expand_ranges everywhere that we use the brace syntax {}. Instead, whenever we are expanding factors, transparently the ranges will be expanded.

• 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

[marcosboger]

Two doubts, open to input:

• Would adding examples to the docs already be enough? Or should we add a new section explaining specifically this?
• Should we also create a more complete parameterized test like test_env_list_expands_ranges for internal sections? Would be kinda redundant as they'll be leveraging same function for expansion, but open to hear opinions