MAINT More fixes to include test/ folder in sdist

[rth]

Followup on #154

Related to #150

Sorry, it looks like previous PR was not enough. Some changes to setup.py were also necessary.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@39ad33b). Click here to learn what that means.
The diff coverage is 54.54%.

@@            Coverage Diff            @@
##             master     #159   +/-   ##
=========================================
  Coverage          ?   93.69%           
=========================================
  Files             ?       22           
  Lines             ?     2585           
  Branches          ?        0           
=========================================
  Hits              ?     2422           
  Misses            ?      163           
  Partials          ?        0

Impacted Files	Coverage Δ
tslearn/__init__.py	75% <100%> (ø)
tslearn/tests/test_estimators.py	89.28% <50%> (ø)

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