Clarify Python version compatibility

[hauntsaninja]

README mentions support for Python 2.6 through 3.5, however astunparse appears to have Python 3.8 support. I noticed, however, that there don't appear to be any tests for the walrus operator or positional-only args.

[hauntsaninja]

But 3.8's Lib/test/test_tools/test_unparse.py doesn't seem to contain tests for those either. I did find one or two random tests common.py doesn't have, which I've opened a PR for at #51

[hauntsaninja]

I've opened a PR with tests for 3.8 features at #52. Hope that's okay! There seems to be precedent with async constructs.