remove const declaration in @pydef which is deprecated in local scope

[marius311]

@pydef declares the class const which if done in the local scope is deprecated in 0.7 / errors on 1.0. I just removed the const. I suppose it could still remain there in the global scope, which would offer slightly better type-stability / performance, but if you're building / calling into a Python class I would argue the overhead is likely to be insignificant in most cases.

[stevengj]

I guess you can still do

const MyClass = PyCall.@pydef_object ....

but in a module setting I guess you couldn't define this as a const global anyway because it wouldn't be precompile-safe.