Fix test failures on tahoe --version and Python 3.7 and 3.8

[jaraco]

While familiarizing myself with the test suite, I ran into some surprising behaviors when trying to run the test suite. In particular, I ran the tests with simply tox, but later realized that because I run with TOX_ENV=python, I was running the Python 2 tests suite against Python 3.8, which led to some failures that were pretty straightforward to fix:

1. The project fails to install on Python 3.7+. I recommend the project should install even on unsupported environments, allowing for testing on those environments.
2. Run the tahoe --version tests on all Pythons, eliminating one point of variance between Python versions.
3. Replace the async keyword with a suitable substitute, allowing the code to pass syntax checks on Python 3.7+.

[jaraco]

To be clear, I understand that Python 3.6 is the target. These changes will add forward compatibility easing development, but acknowledging that Python 3.6 is still the target.

[GitHub <noreply@…>]

In 23e1223c/trunk:

Merge pull request #792 from jaraco/3394.py38-tests

Improve compatibility for later Pythons and restore tahoe command on Python 3

Fixes: ticket:3394