CAS support in FakeVault, raise CasError in client on mismatch, cleanups

[codecov[bot]]

Codecov Report

Merging #136 into develop will increase coverage by 0.04%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #136      +/-   ##
===========================================
+ Coverage    99.91%   99.95%   +0.04%     
===========================================
  Files           29       29              
  Lines         2283     2325      +42     
  Branches       120      128       +8     
===========================================
+ Hits          2281     2324      +43     
+ Misses           1        0       -1     
  Partials         1        1

Impacted Files	Coverage Δ
marathon_acme/clients/tests/helpers.py	100% <ø> (+2.85%)	:arrow_up:
marathon_acme/clients/vault.py	100% <100%> (ø)	:arrow_up:
marathon_acme/tests/fake_vault.py	100% <100%> (ø)	:arrow_up:
marathon_acme/clients/tests/test_vault.py	100% <100%> (ø)	:arrow_up:
marathon_acme/tests/test_fake_vault.py	100% <100%> (ø)	:arrow_up:

[bearnard]

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