As we depend on the cryptography library, and the pip version that comes
with Ubuntu Focal isn't new enough to avoid the following error about a
rust compiler and pre-built wheels, ensure Travis uses an up-to-date
pip.

    error: can't find Rust compiler

    If you are using an outdated pip version, it is possible a prebuilt wheel is available for this package but pip is not able to install from it. Installing from the wheel would avoid the need for a Rust compiler.

    To update pip, run:

        pip install --upgrade pip