Allow selection of implementations (eg CPython, PyPy, Graal)

[DavidCEllis]

I probably want to restrict the implementations that will be used to execute code. For instance GraalPy is incompatible with this module and wouldn't work if it was selected. There's a chance it could get selected automatically so it should probably be excluded (I think currently it may end up excluded by virtue of using an alpha tag).

By default CPython should probably be the only implementation used if unspecified.