Fix Travis builds

[radarhere]

The Travis builds have started failing.

The last successful run of Travis was on July 10.
The first failing run on main was on July 30, with the error

docker: invalid reference format: repository name must be lowercase.

Now, you would think that this is because our repository is named "Pillow".

However, if I switch to "pillow", then the error becomes

Unable to find image 'pillow:latest' locally

docker appears to be trying to find a docker image named "pillow". What I surmise is happening is that when build_wheel passes"build_wheel Pillow" to $build_cmds, docker thinks that "Pillow" is the name of the docker image. It has stopped understanding that the space in the middle does not mean that "BUILD_COMMANDS" is finished, and thinks "Pillow" is a separate argument.

For some reason, this problem doesn't occur in our GitHub Actions builds, or in #367, which continue to pass. So I don't know what has caused docker to behave like this in Travis.

I have found that two changes can workaround this problem.

• I experimented with replacing build_wheel in travis.yml just with build_multilinux aarch64 build_wheel. multibuild would work with just "build_wheel" as the $build_cmds argument... if the $REPO_DIR environment variable was passed through. I created Changed repo_dir variable to be uppercase multi-build/multibuild#511 to try and fix that, but it hasn't been merged. Instead, this PR effectively creates a simplified build_multilinux in travis.yml.
• Continuing with the fact that docker doesn't like spaces in arguments, after that first change, I get

Unable to find image 'pytest-timeout:latest' locally

So I have also changed TEST_DEPENDS from "pytest pytest-timeout" to "pytest-timeout"

[hugovk]

Fun! Thanks for the fix!