Add dirtiness check for refresh token

[bachya]

Describe what the PR does:

This PR adds the ability to check the "dirtiness" of the refresh token (i.e., whether it has been changed or not). This allows applications that store refresh tokens to a filesystem, database, etc. to not constantly:

1. Read from the filesystem/DB
2. Determine whether the refresh token has changed
3. Save the new one to the filesystem/DB

Since the token won't change a large portion of the time, this is wasteful. Instead, at any point, library users can check for themselves:

system.api.refresh_token_dirty
# >>> True

Note that this does create something of an observer effect: by viewing the dirtiness property, we automatically reset it to False.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests is written for the new functionality.
• Update README.md with any new documentation.
• Run tests and ensure 100% code coverage: make coverage (after running make init)
• Ensure you have no linting errors: make lint (after running make init)
• Ensure you have no typed your code correctly: make typing (after running make init)
• Add yourself to AUTHORS.md.

[codecov]

Codecov Report

Merging #15 into dev will not change coverage.
The diff coverage is 100%.

@@         Coverage Diff         @@
##            dev    #15   +/-   ##
===================================
  Coverage   100%   100%           
===================================
  Files         5      5           
  Lines       259    271   +12     
===================================
+ Hits        259    271   +12

Impacted Files	Coverage Δ
simplipy/api.py	100% <100%> (ø)	⬆️

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