fix: Add util for rotating file

[travipross]

Utility function update to make it easier to save more than one copy of locally cached server rates.

Suggestion:

• Wherever server rates are written to disk, call rotate_backups() on the filename just before writing the new version. Note you may have to change when rates are written to disk, as I think you might now need to write to disk now even when rates haven't changed.
• When comparing rates and determining whether or not to notify via discord, check the following conditions (ignore pseudocode):

  last_rates.1.json != last_rates.json # 1. previous two cached rates are different from one another
  current_rates == last_rates.json # 2. current rates have remained constant for at least one full polling period

[FM-17]

Tested, not seeing any errors. I'll create a separate branch if I build off of it.

[github-actions[bot]]

:tada: This PR is included in version 0.6.8 :tada:

The release is available on:

• GitHub release
• v0.6.8

Your semantic-release bot :package::rocket: