Remove f-string usage for Python 3.5 compatibility

[sbesson]

The usage of f-strings breaks Python 3.5 compatibility which affects the ability to run OMERO on some environments. This is a minor change which restores some compatibility.

[joshmoore]

Looks good to me.

[joshmoore]

(I also don't see other hits trying various regexes like

  492  git grep -E '[^"-a-z]f".*"'
  496  git grep -E "[^'-]f'.*'"

)