travis: enable bionic & multi-cpu testing
- [x] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

what does this actually do? (the arch directive) > what does this actually do? (the arch directive)

spins up Ubuntu LXD containers on arm64, s390x (big endian, 64bit mainframe), ppc64el (POWER8 6bit little endian), amd64 metal and lets one build & run code on those architectures, thanks to hardware that Travis got from partners. @jreback  you can see from https://travis-ci.org/pandas-dev/pandas/builds/654862395?utm_source=github_status&utm_medium=notification that all tests on non-x86 are currently failing.

Similarly this is the reason we can't update pandas in Ubuntu 20.04 LTS for the release in april at the moment - https://launchpad.net/ubuntu/+source/pandas/0.25.3+dfsg-5 @xnox we also don't have the capacity to actually have multi arch on *every* build. @xnox is there also infrastructure for building wheels on these platforms? @WillAyd I had a very good experience with https://github.com/joerick/cibuildwheel we would *only* need a single build on each arch, need a PR more like: https://github.com/pandas-dev/pandas/pull/30641