build: add full Python 3 tests to Travis CI
This PR significantly reduces our Travis CI build times and provides better test coverage.
Remove the former, incomplete Travis CI runs on Python 3.6 and 3.7 and replace them with the full 5 stage pipeline running on both Python 2.7 and 3.6.
The Python 3.6 tests pass all runs except Test C++ Suites (py3) as discussed in #29246 (closed) and the Python 3.6 run can be replaced with a Python 3.7 run when #29346 or similar has landed.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines