Provide mechanism to disable conversion to None

[virtuald]

Reference: robotpy/robotpy-commands-v2#23

I do wonder if we should disable conversions to None by default for certain types -- in particular std::function. It wouldn't work in every case, but most of the time a passed in std::function is expected to be valid.

[auscompgeek]

I presume you mean conversion from None?

[virtuald]

words are hard