bump supported Python versions and correct lat2SW doctest

[jGaboardi]

Bumping supporting Python versions in TravisCI to match pysal/pysal#1095

{3.5, 3.6} --> {3.6, 3.7}

[jGaboardi]

Failure does not appear to be do to the {3.6,3.7} upgrade, but a simple failed doctest. This seems to be an issue with how the scipy.sparse object is being returned in the doctest and may be related to the current pysal/pysal build failures (dealing withnumpy/scipy). The failures in pysal/pysal appear to be related to numpy/numpy#10565.

======================================================================
FAIL: Doctest: libpysal.weights.util.lat2SW
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/jGaboardi/libpysal/miniconda/envs/test-env/lib/python3.6/doctest.py", line 2199, in runTest
    raise self.failureException(self.format_failure(new.getvalue()))
AssertionError: Failed doctest test for libpysal.weights.util.lat2SW
  File "/home/travis/build/jGaboardi/libpysal/libpysal/weights/util.py", line 1122, in lat2SW
----------------------------------------------------------------------
File "/home/travis/build/jGaboardi/libpysal/libpysal/weights/util.py", line 1156, in libpysal.weights.util.lat2SW
Failed example:
    w9[0,1]
Expected:
    1
Got:
    array(1, dtype=int8)
----------------------------------------------------------------------
File "/home/travis/build/jGaboardi/libpysal/libpysal/weights/util.py", line 1158, in libpysal.weights.util.lat2SW
Failed example:
    w9[3,6]
Expected:
    1
Got:
    array(1, dtype=int8)

[weikang9009]

Hi @jGaboardi, this seems to be related to NumPy array scalar? After pulling your commits to my local machine and ran doctests locally, I ran into this error:

I am on Mac, python 3.7.3, numpy 1.16.3

[jGaboardi]

The tests for pass locally for me, both for Python 3.6 and 3.7.3 with numpy 1.16.3.

[sjsrey]

----------------------------------------------------------------------
File "/home/serge/Dropbox/p/pysal/src/subpackages/libpysal/libpysal/weights/util.py", line 1156, in libpysal.weights.util.lat2SW
Failed example:
    w9[0,1]
Expected:
    array(1, dtype=int8)
Got:
    1

[sjsrey]

----------------------------------------------------------------------
File "/home/serge/Dropbox/p/pysal/src/subpackages/libpysal/libpysal/weights/util.py", line 1156, in libpysal.weights.util.lat2SW
Failed example:
    w9[0,1]
Expected:
    array(1, dtype=int8)
Got:
    1

The local failures could be due to not replicating the travis envs for the version bump.

[jGaboardi]

So this is a bit strange, correct? The tests pass on locally (for me) and on Travis for 3.6 and 3.7, but fail for you guys.

[sjsrey]

The PR was passing on travis, so I merged.

We can always revert: