Fix flake8 errors with new version >= 3.8

[sbellem]

Fixes #446

f-strings with no placeholder are not allowed, e.g.: f'the color is yellow" must be either f"the color is {color}" or "the color is yellow" ...

variables l are flagged as being ambiguous ... PEP 8 is enforced:

Never use the characters 'l' (lowercase letter el), 'O' (uppercase letter oh), or 'I' (uppercase letter eye) as single character variable names.

In some fonts, these characters are indistinguishable from the numerals one and zero. When tempted to use 'l', use 'L' instead.

I did not make modifications there, but ignored the error by adding # noqa E741.

[codecov[bot]]

Codecov Report

Merging #447 into dev will not change coverage. The diff coverage is 66.66667%.

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  Coverage   77.27842%   77.27842%           
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  Files             50          50           
  Lines           5585        5585           
  Branches         856         856           
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  Hits            4316        4316           
  Misses          1095        1095           
  Partials         174         174