build: add Python 3 tests to Travis CI
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
Based on the lessons learned in #29193, this pull request adds Travis CI test suite named Python 3 is EXPERIMENTAL. These tests are run in allow_failures mode on Python 3.7.1 and they bypasses the Python version checks in ./configure by directly running ./configure.py.