Pyupgrade 3.6: Update syntax with Python 3.6+ features

[EwoutH]

Updated Python code with Python 3.6+ features:

• utf-8 encoding is now the default (PEP 3120, Python 3.0+)
• Replace list comprehensions by Generator Expressions (PEP 289, Python 2.4+)
• Replace yield loop by yield from (PEP 380, Python 3.3+)
• Replace the IOError alias by OSError (PEP 3151, Python 3.3+)
• Remove the default subclass (object) when defining a class
• Define sets with curly braces {} instead of set()
• Remove "r" parameter from open function, which is default

Used pyupgrade 2.29.1 with --py36-plus as starting point for this PR.

[EwoutH]

Thanks for the suggestions, I implemented them, squashed the commits, rebased and signed it.

[adeebshihadeh]

Can you fix the failing tests?

[EwoutH]

I think replacing the list() by [] as @GregorKikelj suggested didn't play well with the lambda, so I reverted it. I did this a bit to quick, and accidentally reset the whole branch. Luckily git reflow is a thing and I recovered it. I couldn't get this PR reopened though, so it's now #23305.

(note to self: be really careful touching git reset --hard)