Upgrade Python syntax with pyupgrade

[hugovk]

Fixes #18.

Ran https://github.com/asottile/pyupgrade/ with --py36-plus.

[codecov]

Codecov Report

Merging #37 into master will not change coverage.
The diff coverage is 0.00%.

@@          Coverage Diff           @@
##           master     #37   +/-   ##
======================================
  Coverage    0.72%   0.72%           
======================================
  Files           9       9           
  Lines         685     685           
======================================
  Hits            5       5           
  Misses        680     680           

Impacted Files	Coverage Δ
pyirf/io/io.py	0.00% <0.00%> (ø)
pyirf/perf/cut_optimisation.py	0.00% <0.00%> (ø)

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