Dropbox api upgrade

[jschneier]

No description provided.

[jschneier]

Supersedes #228 Closes #227 @ZuluPro you are the original author of Dropbox, how does this look?

[codecov-io]

Codecov Report

Merging #273 into master will not change coverage.
The diff coverage is 92.85%.

@@           Coverage Diff           @@
##           master     #273   +/-   ##
=======================================
  Coverage   75.61%   75.61%           
=======================================
  Files          11       11           
  Lines        1542     1542           
=======================================
  Hits         1166     1166           
  Misses        376      376

Impacted Files	Coverage Δ
storages/backends/dropbox.py	97.36% <92.85%> (ø)	⬆️

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[ZuluPro]

@jschneier : Sounds good, but not yet tested