Make Toplevel.copy() deprecation warning more informative

[rpgoldman]

Here's an example of the warning emitted by this line:

/object.py:101: DeprecationWarning: Use sbol3.copy() instead
warnings.warn('Use sbol3.copy() instead', DeprecationWarning)

Suggest the deprecation warning should say what is being deprecated. Propose this modification:

warnings.warn('Toplevel.copy() is deprecated; use sbol3.copy() instead', DeprecationWarning)

[Omarelsherif010]

This seems better. I will go with this issue and implement your suggested modification

@tcmitchell Do I need to be assigned to the issue or I can work on it right away?

[tcmitchell]

@Omarelsherif010 I have assigned this to you. Thanks for picking it up!

[Omarelsherif010]

I think we should close this issue now

[tcmitchell]

I think we should close this issue now

Yes!