Support Python 3.8

[hawkowl]

This mostly requires adding it to CI.

[hawkowl]

Blocking Twisted issue: https://twistedmatrix.com/trac/ticket/9718#ticket

[hawkowl]

Weird test error?

[FAIL]
Traceback (most recent call last):
  File "/home/hawkowl/code/synapse/.tox/py38/lib/python3.8/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
    result = g.send(result)
  File "/home/hawkowl/code/synapse/synapse/util/patch_inline_callbacks.py", line 146, in check_yield_points_inner
    d = gen.send(result)
  File "/home/hawkowl/code/synapse/tests/storage/test__base.py", line 308, in test_double_get
    self.assertEquals(a.func2.cache.cache.pop.call_count, 3)
  File "/home/hawkowl/code/synapse/.tox/py38/lib/python3.8/site-packages/twisted/trial/_synctest.py", line 432, in assertEqual
    super(_Assertions, self).assertEqual(first, second, msg)
  File "/home/hawkowl/.pyenv/versions/3.8.0/lib/python3.8/unittest/case.py", line 912, in assertEqual
    assertion_func(first, second, msg=msg)
  File "/home/hawkowl/.pyenv/versions/3.8.0/lib/python3.8/unittest/case.py", line 905, in _baseAssertEqual
    raise self.failureException(msg)
twisted.trial.unittest.FailTest: 4 != 3

tests.storage.test__base.CacheDecoratorTestCase.test_double_get

[auscompgeek]

Relevant issue: #6200

[hawkowl]

Twisted issue is fixed in the downstream dependency (glyph/automat#118)

[V02460]

I wrote #6213 to address the weird test error. I guess the bug did not materialize because of an implementation detail of Python < 3.8 where different functions share the same reference/are the same object.

[V02460]

Adding Python 3.8 support to CI should now be possible as #6213 was merged.

[richvdh]

we now have CI

[richvdh]

(matrix-org/pipelines#29)