Changes for dropping Py3.5 support

[mastersplinter]

3.5 is end-of-lifed at 2020-09-13 as per the official status page.

Use this Issue to track Python 3.5 specific code that could be dropped after 2020-09-13.

• change string substitutions to f-strings >> new issue replace '%s' string formatting for f-string formatting #273
• drop 3.5 specific pylint exceptions
• improve add_columns doctests to print out result (instead of checking equality).