Can't run simple pytest session in a thread while using labgrid

Hi,
While trying to invoke pytest in a thread with labgrid plugin in use
RuntimeError is raised.
Maybe the exeption should be catched in the module?

Example code:

from threading import Thread
import pytest

def tt():
    pytest.main(['-s', f'--lg-env=../qemu.yaml'])

t = Thread(target=tt, args=())
t.start()
t.join()

Can be worked around by adding this to the thread target function:

loop = asyncio.new_event_loop()
asyncio.set_event_loop(loop)

The line at fault:
https://github.com/labgrid-project/labgrid/blob/d11a5ecc86decd74d3ff86b6fb8b730c9ea4a962/labgrid/remote/client.py#L30

The error log:

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python3.7/threading.py", line 926, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.7/threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "hm.py", line 5, in tt
    pytest.main(['-s', f'--lg-env=../qemu.yaml'])
  File "/home/marek/.local/lib/python3.7/site-packages/_pytest/config/__init__.py", line 105, in main
    config = _prepareconfig(args, plugins)
  File "/home/marek/.local/lib/python3.7/site-packages/_pytest/config/__init__.py", line 258, in _prepareconfig
    pluginmanager=pluginmanager, args=args
  File "/home/marek/.local/lib/python3.7/site-packages/pluggy/hooks.py", line 286, in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
  File "/home/marek/.local/lib/python3.7/site-packages/pluggy/manager.py", line 93, in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
  File "/home/marek/.local/lib/python3.7/site-packages/pluggy/manager.py", line 87, in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
  File "/home/marek/.local/lib/python3.7/site-packages/pluggy/callers.py", line 203, in _multicall
    gen.send(outcome)
  File "/home/marek/.local/lib/python3.7/site-packages/_pytest/helpconfig.py", line 90, in pytest_cmdline_parse
    config = outcome.get_result()
  File "/home/marek/.local/lib/python3.7/site-packages/pluggy/callers.py", line 80, in get_result
    raise ex[1].with_traceback(ex[2])
  File "/home/marek/.local/lib/python3.7/site-packages/pluggy/callers.py", line 187, in _multicall
    res = hook_impl.function(*args)
  File "/home/marek/.local/lib/python3.7/site-packages/_pytest/config/__init__.py", line 836, in pytest_cmdline_parse
    self.parse(args)
  File "/home/marek/.local/lib/python3.7/site-packages/_pytest/config/__init__.py", line 1044, in parse
    self._preparse(args, addopts=addopts)
  File "/home/marek/.local/lib/python3.7/site-packages/_pytest/config/__init__.py", line 992, in _preparse
    self.pluginmanager.load_setuptools_entrypoints("pytest11")
  File "/home/marek/.local/lib/python3.7/site-packages/pluggy/manager.py", line 299, in load_setuptools_entrypoints
    plugin = ep.load()
  File "/home/marek/.local/lib/python3.7/site-packages/importlib_metadata/__init__.py", line 95, in load
    module = import_module(match.group('module'))
  File "/usr/lib/python3.7/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "/home/marek/.local/lib/python3.7/site-packages/_pytest/assertion/rewrite.py", line 152, in exec_module
    exec(co, module.__dict__)
  File "/home/marek/.local/lib/python3.7/site-packages/labgrid/pytestplugin/__init__.py", line 1, in <module>
    from .fixtures import pytest_addoption, env, target
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "/home/marek/.local/lib/python3.7/site-packages/_pytest/assertion/rewrite.py", line 152, in exec_module
    exec(co, module.__dict__)
  File "/home/marek/.local/lib/python3.7/site-packages/labgrid/pytestplugin/fixtures.py", line 6, in <module>
    from ..remote.client import UserError
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "/home/marek/.local/lib/python3.7/site-packages/_pytest/assertion/rewrite.py", line 152, in exec_module
    exec(co, module.__dict__)
  File "/home/marek/.local/lib/python3.7/site-packages/labgrid/remote/client.py", line 30, in <module>
    txaio.config.loop = asyncio.get_event_loop()
  File "/usr/lib/python3.7/asyncio/events.py", line 644, in get_event_loop
    % threading.current_thread().name)
RuntimeError: There is no current event loop in thread 'Thread-1'.

Reproduce the threaded pytest.main example, then inspect labgrid/remote/client.py at line 30 and the import path through labgrid/pytestplugin/fixtures.py. Verify the behavior with and without the explicitly created asyncio event loop; done means a pytest session started from a thread no longer raises the reported RuntimeError.