Python 3.8 support

[willfurnass]

Python 3.8 support required PR #150 to be merged, which has now been done.

Also ensures that all tests run with Python 3.6, 3.7 and 3.8 but coverage only calculated and uploaded for Python 3.7 (could not do this for just 3.8 instead as coverage doesn't work with Python 3.8 yet - see https://github.com/nedbat/coveragepy/issues/772).

[coveralls]

Coverage remained the same at 11.314% when pulling a26d650bfc670f036f98ad60f3b085dc1d7ac050 on willfurnass:py38 into 2c8bd97824384c9a4cc3ec87f98a4553c1471bf6 on robclewley:master.

[willfurnass]

Hmm, odd that Travis CI jobs now only fail when Python=3.7 and COVERAGE="yes". NB those tests result in multiple occurrences of:

...
           if self._parameterized:
>               self.indepvararray.resize(new_len)
E               ValueError: cannot resize an array that references or is referenced
E               by another array in this way.  Use the resize function
...