Pytest compatibility

[Peque]

These changes implement some pytest compatibility in tests. Note that, in order to keep compatibility with older test-running methods, an if __name__ == '__main__' section has been created in all test files to force execution when running tests directly with the interpreter.

This is really inconvenient and I am against it, as adding tests would imply the need to add explicit calls within that section. I guess it could be okay as a transition, though. And it is also impossible to avoid as long as pytest is not adopted as a dependency for running (adapted) dill tests. 😉

[mmckerns]

Can you (1) update this PR, (2) delete test_file.py from this PR... or don't make it explicitly depend on the import of pytest and make sure that the .txt file artifact is deleted?

Otherwise, it's ready to PR, and I'll address the conversion of the other tests myself.

[mmckerns]

@Peque: Thanks, btw, for pushing on this and the other PRs. I'm working through it -- I will also see what I can do to address the issue that you are most interested in for pypy (i.e. #175).

[Peque]

@mmckerns Rebased. Removed pytest dependency and fixed test_file.py clean-up/teardown (d914795).

[mmckerns]

Thanks, I'll convert the rest to pytest from here.