Ignore whitespace in library section lookup

[jjlawren]

Description

Ignores whitespace when looking up a library section using Library.section().

This handles a workaround used by some users to reorder libraries by prepending spaces to the library section name.

Example: https://old.reddit.com/r/PleX/comments/9adjku/change_order_of_libraries/e4v5q5c/

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable