modernize formatting code for python 3.6+

[kosack]

Since we dropped support for Python 3.5, we can now use some of the features of 3.6 everywhere. This PR upgrades the use of str.format to use f-strings instead (cleaner). The edits were not made manually, rather I used the pyupgrade refactoring tool (https://github.com/asottile/pyupgrade)

[codecov]

Codecov Report

Merging #944 into master will not change coverage.
The diff coverage is 63.82%.

@@           Coverage Diff           @@
##           master     #944   +/-   ##
=======================================
  Coverage   78.86%   78.86%           
=======================================
  Files         191      191           
  Lines       10966    10966           
=======================================
  Hits         8648     8648           
  Misses       2318     2318

Impacted Files	Coverage Δ
ctapipe/utils/CutFlow.py	94.91% <ø> (ø)	⬆️
ctapipe/core/traits.py	95.65% <ø> (ø)	⬆️
ctapipe/io/toymodel.py	0% <ø> (ø)	⬆️
ctapipe/image/muon/muon_reco_functions.py	66.66% <0%> (ø)	⬆️
ctapipe/tools/extract_charge_resolution.py	73.91% <0%> (ø)	⬆️
ctapipe/io/targetioeventsource.py	14.72% <0%> (ø)	⬆️
ctapipe/io/array.py	50% <0%> (ø)	⬆️
ctapipe/tools/bokeh/file_viewer.py	72.33% <0%> (ø)	⬆️
ctapipe/coordinates/representation.py	80.55% <0%> (ø)	⬆️
ctapipe/core/container.py	67.32% <0%> (ø)	⬆️
... and 29 more

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[kosack]

This is pretty uncontroversial I think and ready for review - the poor codecov is unrelated - it just detects all of the coverage problems in the affected files

[kosack]

ah, it seems codacy still is not treating this as modern python code (hence it now complains about the use of f-strings). I think we can ignore that for now and I'll try to see if there is a way around it (they claim it's fixed https://github.com/codacy/codacy-pylint/issues/41 , but I seems not to be)

[kosack]

It seems no option I try makes codacy understand that this is python3.6. I could simply disable Prospector, perhaps. Even disabling the error F999 seems to have no effect.

[kosack]

I'll contact their support, but for now let's just accept this as is.

[kosack]

I received an answer from Codacy, and it seems they are still using an outdated version of Prospector that doesn't support python 3.6+ syntax. They suggested just disabling that pattern (Prospector_F999), so I did so. Otherwise this PR is ready to go