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 nedbat/coveragepy#772).

[coveralls]

Coverage remained the same at 11.314% when pulling a26d650 on willfurnass:py38 into 2c8bd97 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
...