Simplify dict iterations from Python 2 to 3 migration

[Avasam]

• Python 2 to 3 automatic migration tooling added a bunch of unneeded list calls when the dict_keys/dict_values/dict_items object is immediately iterated upon.
• Similarly, removed redundant calls to iter on dict_keys/dict_values/dict_items
• Iterate over values only when the keys aren't used
• Iterate over keys only when the values aren't used
• Chain iterable instead of generating and concatenating multiple lists (only for lines affected by above changes)
• Use sets instead of dicts when the value isn't used (only for lines affected by above changes and only for non-public names)
• Uses comprehensions when relevant (only for lines affected by above changes)

Equivalent adodbapi PR: #2332