MAINT: Address Pylint issues

[carlwilson]

• no functional changes;
• used of f'' string rather than str.format();
• better used of constants for strings, though not perfect;
• better code documentation, though far from perfect;
• test adjustments to match, though no pylint on test code;
• use of generation expressions rather than list comprehensions; and
• various occurrences of minor issues:
  • unused variables/parameters/imports;
  • overlong lines; and
  • redundant braces and else statements.

[darrendignam]

The changes are welcomed, settings a more consistent style going forward.

The only quibble I would have is the declaration of constants throughout the project.
One I see at least twice now is:
METS: str = 'METS.xml'

If we have string constants appearing in multiple modules, perhaps we should consider using a 'constants.py' module to declare these strings in one location.

[carlwilson]

The only quibble I would have is the declaration of constants throughout the project. One I see at least twice now is: METS: str = 'METS.xml'

My intention is to gather these up once factored out. Next pass...