Python deprecation warnings: collections, imp, logging.warn

[andersk]

From the py.test output (as well as the output of other projects using hyper, if warnings are enabled):

=============================== warnings summary ===============================
hyper/http11/connection.py:13
hyper/http11/connection.py:13
  /hyper/hyper/http11/connection.py:13: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, and in 3.9 it will stop working
    from collections import Iterable, Mapping

/tmp/ve/lib/python3.8/site-packages/hyperframe/flags.py:14
  /tmp/ve/lib/python3.8/site-packages/hyperframe/flags.py:14: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, and in 3.9 it will stop working
    class Flags(collections.MutableSet):

hyper/common/headers.py:13
  /hyper/hyper/common/headers.py:13: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, and in 3.9 it will stop working
    class HTTPHeaderMap(collections.MutableMapping):

test/test_import.py:2
  /hyper/test/test_import.py:2: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
    import imp

test/test_hyper.py::TestHyperConnection::test_closed_connections_are_reset
  /hyper/hyper/http20/connection.py:465: DeprecationWarning: The 'warn' method is deprecated, use 'warning' instead
    log.warn("GoAway frame could not be sent: %s" % e)

test/test_hyper.py::TestServerPush::test_reset_pushed_streams_when_push_disabled
test/test_hyper.py::TestUpgradingPush::test_reset_pushed_streams_when_push_disabled
  /hyper/hyper/http20/connection.py:841: DeprecationWarning: The 'warn' method is deprecated, use 'warning' instead
    log.warn(

-- Docs: https://docs.pytest.org/en/latest/warnings.html