Remove CI testing for Python 2.7, add 3.8 on Travis

[alubbock]

As per #463 and #414, the next version on PySB will remove Python 2.7 support. This PR removes Python 2.7 testing on Travis and Anaconda. Travis will test on 3.6, 3.7, and 3.8; Appveyor on 3.6 and 3.7 (3.8 not yet available by default on Appveyor).

[jmuhlich]

💯

[codecov-io]

Codecov Report

Merging #479 into master will decrease coverage by 0.29%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #479     +/-   ##
=========================================
- Coverage   80.11%   79.81%   -0.3%     
=========================================
  Files         101      101             
  Lines       10358    10586    +228     
=========================================
+ Hits         8298     8449    +151     
- Misses       2060     2137     +77

Impacted Files	Coverage Δ
pysb/simulator/scipyode.py	69.66% <0%> (-19.76%)	⬇️
pysb/tests/test_simulator_scipy.py	92.99% <0%> (-5.33%)	⬇️
pysb/tests/test_examples.py	96.87% <0%> (-3.13%)	⬇️
pysb/importers/sbml.py	76.92% <0%> (-2.57%)	⬇️
pysb/tests/test_simulator_bng.py	100% <0%> (ø)	⬆️
pysb/tests/test_simulator_kappa.py	100% <0%> (ø)	⬆️
pysb/tests/test_simulator_stochkit.py	100% <0%> (ø)	⬆️
pysb/tests/test_kappa.py	100% <0%> (ø)	⬆️
pysb/tests/test_sensitivity_analysis.py	100% <0%> (ø)	⬆️
pysb/tests/test_integrate.py	100% <0%> (ø)	⬆️
... and 19 more

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