Improved coverage

[ErikBjare]

Merge after #57

[codecov-io]

Codecov Report

Merging #62 into master will increase coverage by 1.14%.
The diff coverage is 96.82%.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   93.97%   95.11%   +1.14%     
==========================================
  Files          33       33              
  Lines        1527     1536       +9     
  Branches      230      233       +3     
==========================================
+ Hits         1435     1461      +26     
+ Misses         47       44       -3     
+ Partials       45       31      -14

Impacted Files	Coverage Δ
aw_datastore/storages/abstract.py	96.87% <ø> (ø)	⬆️
aw_datastore/storages/sqlite.py	98.6% <100%> (+0.02%)	⬆️
aw_datastore/storages/mongodb.py	97.84% <100%> (+4.44%)	⬆️
aw_transform/merge_events_by_keys.py	100% <100%> (+17.39%)	⬆️
aw_datastore/datastore.py	100% <100%> (+4.16%)	⬆️
aw_datastore/storages/peewee.py	99.21% <100%> (+0.01%)	⬆️
aw_datastore/storages/memory.py	94.2% <80%> (+3.29%)	⬆️
... and 3 more

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[johan-bjareholt]

For a peewee->sqlite database migration to start these are the criterias:

• Start a Datastore object with sqlite backend
• There does not exist a previous sqlite database
• There does exist a previous peewee.v2.db database

So the solution to avoid a migration during testing would be to run the datastore tests with sqlite before running them with peewee.