Runtime: work around 1s delay added when using twisted reactor's callLater.

Since wxPython4, using wxReactor from non-main thread was producing
exceptions in wxWidget's C++ code. Then reactor.run() was called from
main thread, and runtime's worker was delegating calls to reactor
with callLater(0, callable).

While this worked perfectly with wxReactor, it did introduce an unexplained
1 second delay to each worker call when using nomal linux reactors
(i.e. without wxPython). As a workaround reactor runs in a thread when using
twisted without wxPython

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# opcua/__init__.py

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from __future__ import absolute_import

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from .client import OPCUAClient

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# class RootClass(object):

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#     XSD = """<?xml version="1.0" encoding="ISO-8859-1" ?>

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#     <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

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#     </xsd:schema>

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#     """

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#     CTNChildrenTypes = [("OPCUAClient", OPCUAClient, "OPCUA Client")]

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#