Add Cython language_level directive to _cantera.pyx

[bryanwweber]

This fixes the warnings generated by recent versions of Cython that
the language_level will be changed in the future. By setting this
directive, all the code in the .pyx files should be written in
Python 3 syntax. This required several changes to the import
syntax in the files to fix relative vs. absolute imports

See https://stackoverflow.com/questions/54900723/what-does-language-level-in-setup-py-for-cython-do for more information about this setting

[speth]

The more directly equivalent way to write this would be from . import interrupts, right?

[bryanwweber]

Yes, I think so, although since its just one function I didn't see the need to keep the namespace. Since that's the only function in that module and that's the only place its used, I thought about removing the module entirely. But since no_op is pure Python, putting it into the .pyx file would cause it to be compiled and expect the type annotations, right?

[speth]

I guess it's the balance between just one function and the fact that it's only used one time -- the form you used certainly makes sense in the case where the function is used many times.

The reason for this module existing is written in the docstring for the no_op function: "a pure Python function is needed in order for KeyboardInterrupt events to be captured.". I'm not sure if there are any additional considerations regarding type annotations.

[bryanwweber]

So if the function were in the .pyx file, it would be compiled and couldn't be used as an interrupt, got it. Thanks!