This commit migrates the font tests away from the bots. Not only are the
font tests broken on the Windows bot since some time, they also run on
Python 2 (end of life since January 2020) and `ttx` 3.19.0 (released in
November 2017). The latter is installed via a submodule, which requires
more complicated logic for finding and running `ttx`.
We solve the issues by implementing a modern workflow that installs the
most recent stable Python and `ttx` (`fonttools` package) versions. This
simplifies the `ttx` driver code as well because it can now assume `ttx`
is available on the path (just like we do for e.g. `node` invocations).
GitHub Actions takes care of creating a virtual environment with
`fonttools` in it so that the `ttx` entrypoint is available. Locally
the font tests can be run in a similar way by creating and sourcing a
virtual environment with `fonttools` in it before running the font
tests, and a README file is included with instructions for doing so.
