Minor Python 2 to Python 3 changes

[carlthome]

Very minor nitpicks but perhaps nice to keep the syntax fresh since Python 3.8 is now the latest non-EOL version.

1. Drop utf-8 encoding header
2. Use f-strings
3. Use set comprehension, set literals, dict comprehensions, instead of converting lists

Nothing major and not urgent to fix. And TBH arguably not worth merging in case this breaks unexpected monkey patches in the wild. This is just me sitting at an airport with a coffee and reading the source. 😄

[carlthome]

Ah, so black is enforced on the source code now too? That's nice!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
mir_eval/display.py	75.00%	1 Missing :warning:

Flag	Coverage Δ
unittests	85.46% <94.44%> (-10.28%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
mir_eval/chord.py	99.20% <100.00%> (ø)
mir_eval/hierarchy.py	100.00% <ø> (ø)
mir_eval/io.py	91.95% <100.00%> (-4.03%)	:arrow_down:
mir_eval/key.py	97.67% <ø> (ø)
mir_eval/pattern.py	98.69% <100.00%> (ø)
mir_eval/segment.py	98.00% <ø> (ø)
mir_eval/separation.py	9.49% <ø> (-89.83%)	:arrow_down:
mir_eval/tempo.py	100.00% <100.00%> (ø)
mir_eval/util.py	98.01% <100.00%> (-0.76%)	:arrow_down:
mir_eval/display.py	77.05% <75.00%> (-8.58%)	:arrow_down: