The current version of ttx that we use is over five years old; see https://github.com/fonttools/fonttools/tree/8388a2e37ce349dac6555bb824c82723e3b65fbf (the current submodule commit). We should really update to the latest release, not only to be up-to-date but also because our version of ttx requires Python 2 which has been discouraged for a long time now and is (finally) out of support.

This issue has come up locally when I ran gulp fonttest and two tests failed. It turns out our script spawns python to run ttx, but python on modern Linux systems refers to Python 3. For Python 2, the python2 command must be used. When I changed this, the tests passed.

This will require that we have Python 3 on the bots, which is also highly recommended nowadays and might already be there.