eliminate "global const foo = bar" from __init__

[stevengj]

This eliminates the global const assignments from __init__, since this is deprecated and doesn't play well with precompilation (JuliaLang/julia#12010).

[coveralls]

Coverage decreased (-0.4%) to 58.729% when pulling 6547543 on noglobalconst into 3ac6ca8 on master.

[coveralls]

Coverage increased (+0.007%) to 59.094% when pulling 4be9ca3 on noglobalconst into 3ac6ca8 on master.

[cstjean]

I don't know enough about PyCall for a detailed review, but this looks OK.

I wouldn't be surprised if some libraries refer PyCall.pynothing... I know it's not exported, but maybe it should be.

[stevengj]

According to a grep of all registered packages, no one is using pynothing outside of PyCall itself.