[tahoe-lafs-trac-stream] [Tahoe-LAFS] #3394: Fix test failures on tahoe --version and Python 3.7 and 3.8

[Tahoe-LAFS]

#3394: Fix test failures on tahoe --version and Python 3.7 and 3.8
-------------------------+----------------------------------
 Reporter:  jaraco       |          Owner:
     Type:  enhancement  |         Status:  new
 Priority:  normal       |      Milestone:  Support Python 3
Component:  code         |        Version:  n/a
 Keywords:               |  Launchpad Bug:
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 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.
 1. Run the `tahoe --version` tests on all Pythons, eliminating one point
 of variance between Python versions.
 1. Replace the `async` keyword with a suitable substitute, allowing the
 code to pass syntax checks on Python 3.7+.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3394>
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