Make decompiler and transformers optional

[Kentzo]

If I understand correctly, both decompiler and transformers are optional to the main functionality of the package: inspection and modification of code objects.

I propose to make both packages optional (e.g. via extras_require).

[llllllllll]

What would the benefit of this be?

[Kentzo]

The idea is to break dependency between generic core and utility functions that depend on specific Python. Version on codetransforms is currently broken for Python 3.6 because of them. Also see #60.

Perhaps they can be made optional (no import in __init__)?

[llllllllll]

The 3.6 issue has already been resolved on master. #57

[Kentzo]

Are there precautions against further opcode changes?

It would be unfortunate to end up with incompatible package because of this.

[llllllllll]

Not currently, is there a reason to be this worried about codetransformer breaking on new versions of Python. Currently we do not support new versions of python right away regardless of how codetransformer is packaged.

[Kentzo]

Given what this package tries to accomplish it's indeed impossible to give any promises since CPython does not give them in the first place. However, certain parts of CPython were proven to be more stable than other.

I do not see how users benefit of automatic import of utility classes that are more fragile than the rest of the package.