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

.travis.yml

@@ -1,4 +1,6 @@
-language: minimal
+os: linux
+dist: focal
+language: python
 
 branches:
   only:
@@ -7,24 +9,25 @@ branches:
 jobs:
   include:
     - name: Linux | x86_64 + i686 | Python 3.6
-      language: python
       python: 3.6
       services: docker
       env: PYTHON=python
 
     - name: Linux | arm64 | Python 3.6
-      language: python
       python: 3.6
       services: docker
       arch: arm64
       env: PYTHON=python
 
     - name: Linux | ppc64le | Python 3.6
-      language: python
       python: 3.6
       services: docker
       arch: ppc64le
-      env: PYTHON=python
+      env:
+        - PYTHON=python
+        # skip test_manylinuxXXXX_only, it uses too much disk space
+        # c.f. https://travis-ci.community/t/running-out-of-disk-space-quota-when-using-docker-on-ppc64le/11634
+        - PYTEST_ADDOPTS='-k "not test_manylinuxXXXX_only"'
 
     - name: Windows | x86_64 | Python 3.6
       os: windows
@@ -36,18 +39,12 @@ jobs:
       env:
         - PYTHON=C:\\Python36\\python
 
-    - &linux_s390x_36
-      name: Linux | s390x | Python 3.6
-      language: python
+    - name: Linux | s390x | Python 3.6
       python: 3.6
       services: docker
       arch: s390x
       env: PYTHON=python
 
-  allow_failures:
-    # must repeat the s390x job above exactly to match
-    - *linux_s390x_36
-
 install: $PYTHON -m pip install -e ".[dev]" pytest-custom-exit-code
 
 script: $PYTHON ./bin/run_tests.py

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