build: test Python 3.6 and 3.7 on Travis CI
Add a Python 3.6 experimental run to Travis CI because the Python 3.7 run fails early because async is a reserved word in Python >= 3.7.
help wanted:
- How can we get the Python 3.6 build to complete before timing out (50 minutes)?
- How can we fix the async issue on Python >= 3.7?
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines