Mbed CLI 1 parses the base requirements.txt and attempts to
automatically install missing python libraries for you. Unfortunately,
it's requirements parser is not as capable as the one shipping with pip.
In particular, we are unable to express different version of libraries
conditional on the version of Python being used.

    ---
    [mbed] ERROR: Unknown Error: Unsupported environment marker:  python_version < '3.6'

Remove the direct dependency on prettytable from requirements.txt, as
mbed-os-tools depends on prettytable and we can pick up the dependency
(as properly expressed conditionally on the Python version) from there,
without having to use the limited requirements parser in Mbed CLI 1.