support 21st century 4 digit year strings for Daterange

[kenny-lee-1]

This PR:

• allows 4 digit year strings for the 21st century to be made as Daterange objects
• date_range only supports up to year 2030 #61

Notes:

• was found after a test in our era codebase made a daterange 5 years in the future.
• the rubocop issue should be addressed eventually as this is an public repo

[ollietulloch]

Thanks @kenny-lee-1, LGTM.

The rubocop issue was not introduced by this PR and should be addressed separately