python 2/3 compatibility changes

[orbeckst]

• address #12 (logger.warning) and #11 (changes in scipy.integrate)
• test all supported python versions (under Linux)

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@41f99e0). Click here to learn what that means. The diff coverage is 90.69%.

@@            Coverage Diff            @@
##             master      #16   +/-   ##
=========================================
  Coverage          ?   70.18%           
=========================================
  Files             ?        5           
  Lines             ?      597           
  Branches          ?       77           
=========================================
  Hits              ?      419           
  Misses            ?      135           
  Partials          ?       43           

Impacted Files	Coverage Δ
src/numkit/timeseries.py	61.41% <60.00%> (ø)
src/numkit/integration.py	59.09% <80.00%> (ø)
src/numkit/__init__.py	100.00% <100.00%> (ø)
src/numkit/fitting.py	84.46% <100.00%> (ø)
src/numkit/observables.py	78.48% <100.00%> (ø)

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