Switch Travis CI test configuration to pytest and add matplotlib tests

[jthielen]

As discussed in #947, this PR switches Travis CI to use pytest for Pint's test suite instead of unittest. As a motivating example, basic tests for pint.matplotlib are added (which closes #897).

• Closes Implement tests for matplotlib functionality #897, towards Switching to pytest for unit testing framework; Use doctest more extensively #947 (but does not fully resolve it)
• Executed black -t py36 . && isort -rc . && flake8 with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
• Added an entry to the CHANGES file

[jthielen]

The switch to pytest seems to be working great, but the matplotlib tests are not matching the baselines I created locally. I'll be taking a look at those, and once I can figure out what is wrong with them, this should be ready for review.

[jthielen]

All the tests are passing now, so this should be ready for review and merge.

[hgrecco]

bors r+

[hgrecco]

Just as a a reminder for the future. Now that pytest is available we could streamline some of the existing tests to make good use of it.

[bors]

Build succeeded

• continuous-integration/travis-ci/push

[westurner]

👍

@pytest.mark.parametrize might make it easier to identify the test failures by creating named test cases for each combination.

ParametrizedTestCase:

https://github.com/hgrecco/pint/blob/2503f00762d1bc159ab196e2de93963f257629dd/pint/testsuite/parameterized.py

pytest.mark.parametrize:

https://github.com/pytest-dev/pytest/blob/8be9684ab23220cc85abb06e06414961667340b3/src/_pytest/python.py#L918-L922

https://github.com/pytest-dev/pytest/blob/8be9684ab23220cc85abb06e06414961667340b3/src/_pytest/python.py#L987-L990 :

       # create the new calls: if we are parametrize() multiple times (by applying the decorator
       # more than once) then we accumulate those calls generating the cartesian product
       # of all calls