Drop Python 3.5 from Travis tests.

[moorepants]

• Support Python 2.7 and 3.6-3.7
• Oldest (from 18.04) will not build with Python 3.8.

[moorepants]

I can't figure out why travis ci isn't always triggered and no longer displays on the commits.

[moorepants]

Figured it out! I could have comments on the if statement lines.

[moorepants]

On the Python 2.7 oldest build I"m getting:

$ THEANO_FLAGS='gcc.cxxflags="-march=core2"' nosetests -v --with-coverage --cover-package=pydy

Traceback (most recent call last):

  File "/home/travis/miniconda/envs/test-env/bin/nosetests", line 6, in <module>

    from nose import run_exit

  File "/home/travis/miniconda/envs/test-env/lib/python2.7/site-packages/nose/__init__.py", line 3, in <module>

    from nose.exc import SkipTest, DeprecatedTest

  File "/home/travis/miniconda/envs/test-env/lib/python2.7/site-packages/nose/exc.py", line 8, in <module>

    from nose.plugins.skip import SkipTest

  File "/home/travis/miniconda/envs/test-env/lib/python2.7/site-packages/nose/plugins/skip.py", line 9, in <module>

    from nose.plugins.errorclass import ErrorClass, ErrorClassPlugin

  File "/home/travis/miniconda/envs/test-env/lib/python2.7/site-packages/nose/plugins/errorclass.py", line 130

    class ErrorClassPlugin(Plugin, metaclass=MetaErrorClass):

                                            ^

SyntaxError: invalid syntax

Not sure why.

[moorepants]

Looks like nose 1.3.7 is marked for Python >=3.6: https://github.com/conda-forge/nose-feedstock/blob/master/recipe/meta.yaml

Not sure why, as 1.3.7 came out in 2015!

[moorepants]

The nose issue seems to be fixed. There were some changes in the nose-feedstock at conda-forge that affected things previously.

[moorepants]

The failures have to do with conda's inability to solve a set of packages. Going to merge as is and address that with the #432