Fix Travis CI tests on ppc64le/s390x (#689)

* Disable manylinux only tests on Travis CI ppc64le

Those tests are taking too much disk space for the Travis CI runner right now.
c.f. https://travis-ci.community/t/running-out-of-disk-space-quota-when-using-docker-on-ppc64le/11634

* Use focal to run aarch64/ppc64le/s390x jobs on Travis CI

This removes allow_failures from the Travis CI s390x job

* Use `focal` distribution for all Travis CI configurations

* Revert "Disable manylinux only tests on Travis CI ppc64le"

This reverts commit d0a09c689c058985c7dcb60b1e11a85a7bfce5e0.

* Skip test_manylinuxXXXX_only on Travis CI ppc64le

* Add a comment on why test_manylinuxXXXX_only test is skipped

examples/travis-ci-deploy.yml

@@ -1,6 +1,8 @@
 # As written, this configuration will build your wheels on every
 # commit, but will only push to PyPI on tagged commits.
 
+os: linux
+dist: focal
 language: python
 
 jobs:

examples/travis-ci-minimal.yml

@@ -1,3 +1,5 @@
+os: linux
+dist: focal
 language: python
 
 jobs:

examples/travis-ci-test-and-deploy.yml

@@ -5,6 +5,8 @@
 # repo is yours (e.g. it won't run on a Pull Request). For convenience, a source
 # distribution is also created.
 
+os: linux
+dist: focal
 language: python
 python:
   - 3.6